Under the Federal Rules of Evidence (FRE), can expert witnesses talk about something that was not included in their deposition transcript but is related to the case at hand?
The Federal Rules of Civil Procedure have little to say about the admissibility of expert testimony. Under the Federal Rules of Evidence, though, it depends on what "something" is.
The fact that a statement was not made during a deposition is not a reason to exclude expert testimony. But the expert's testimony is still subject to all the same rules as any other testimony, so it could be excluded if it were irrelevant, if it violated the rule against hearsay, or if it was speculation.
Most importantly, though, is the question of whether the statement is an opinion. Opinion testimony is generally inadmissible, but there are special exceptions for expert testimony. FRCP 26 does require those opinions to be disclosed in an expert report, though it doesn't, on its face, make any undisclosed opinions inadmissible.
If that requirement is satisfied, though, it's probably not material that the expert didn't discuss it in a deposition. A rule to the contrary could cause all manner of problems, as a party could simply cancel a deposition to prevent an expert from testifying.