Not in England and Wales, because the Judge cannot give testimony of any type.
An expert witness gives hearsay evidence (that is to say, he gives an opinion, not an eye-witness's evidence): necessarily so, because he was not present at the scene of the crime (or, in a civil case, he did not see the events at issue occur). But an expert is the principal exception to the hearsay rule, i.e. the rule that only eye-witness testimony can be given in evidence.
In England it is rare, today, for a jury to sit in a civil case: mainly, juries are now only used in criminal cases. But I think it is nevertheless worth making the point that the rules of evidence are different in civil cases; whereas in criminal trials the rule against hearsay evidence is much more strictly adhered to.
In a civil case, the Judge performs also those functions which, in a criminal case, are performed by the jury. So it is easier, perhaps, to understand the position by considering a civil case: there, the Judge is also the Jury, so it is readily apparent that it would be improper for the Judge to give evidence of any type.
The function of a jury is to decide questions of fact in the case, whereas the function of the Judge is to decide questions of law. This requires the court (composed both of judge and jury) to sit impartially between the two parties to the case: thus the judge can no more give evidence than can the jury.
But the function of the expert witness is to give evidence, usually medical evidence: evidence of a type that requires some degree of professional training, and so falls outside the ordinary knowledge of a jury. The Judge too, like the jury, will simply not have specialist medical or scientific knowledge, and so will not be in a position to give expert evidence.
In regard to points of law, the first thing to understand is that in England a jury has the right to ask the Judge questions: in a sense, the jury is entitled to cross-examine the Judge with regard to points of law. But that is the only resemblance between the role of Judge and the role of Expert Witness.
Also, in England, both the Barrister representing the police, and the Barrister representing the Defendant, are entitled to address the Jury on points of law. It is not solely the Judge who does so. The Jury then decides for itself which of the three (often differing) views on the law it wishes to follow. But the important point to bear in mind is that the Jury is not being briefed on the applicable law by only the Judge: it gets three points of view, not one. And what the outcome in the case is will often depend on what view the jury takes of the eye-witness evidence (i.e. which of the eye-witnesses it believes), and hence may have nothing to do with anything the Judge says.
It is a mistake to suppose that, in England, the Judge gives instructions to the Jury. He does not. His summing-up summarises the evidence given in the case, and then addresses the main points of law which the case has raised. But the Judge is often only setting out legal alternatives: he is telling the jury what the prosecutor must prove, since certain differences do exist between - for instance - what amounts to murder, and what does not but might nevertheless amount to manslaughter (in the USA, termed 2nd-degree murder).
In England, a Judge has the power to instruct the jury on only one matter: if he is convinced that in the circumstances no conviction could possibly be safe, he can order the jury to acquit the defendant. But in all other respects he can only offer guidance, not instructions.
For deliberate reasons of public policy, the expert witnesses are independent not only of the parties to the case (in a criminal trial, this will be the police and the accused), but also are independent of the court too. The experts are, for instance, chosen by the lawyers acting for each party in the case, never by the court.
The Judge's function is to decide whether someone is suitably qualified to act as an expert witness (if the Barristers disagree on this, they typically ask the Judge for a ruling). Clearly, therefore, a judge cannot also be an expert witness: that would violate the essential distinction between the judge's function, which is to choose, and the expert's role, which in this respect is simply to be chosen (or, occasionally, to be rejected).
Hopefully, this throws some light on the issue of why the Judge cannot in any sense be seen as an expert witness.
ADDENDUM : In England, in a very recent change, by Act of Parliament under the last Labour government, Parliament abolished the double jeopardy rule, in effect allowing another court to later set-aside the Jury's verdict (i.e. in the case of an acquital). This permits a defendant to be tried for a second time for the same offence. But it would be quite wrong to suggest this change was not contraversial.
Also, in England the deliberations of a Jury are secret. No politician, Judge, journalist, nor any other person, is entitled to enquire subsequently into why the jury reached the decision it did; nor is a member of the jury permitted to make public statements, or give press interviews, about those deliberations. What is said in the jury room is confidential for all time, in compliance with the public policy objectives of certainty and finality: the preventing of the parties to the case from re-opening it by the backdoor, by bringing into question the proceedings of the jury. Just as proceedings in Parliament cannot be litigated in any court, neither can proceedings in a jury room.