If a bank sends me a contract to sign to apply for a credit card, can I scan the contract, modify it, and then mail it to the bank?
The bank has made you an offer to contract with you: you can accept that offer by signing it and sending it back unamended.
You can make the bank a counter-offer by amending the document, signing it and sending it to them for their consideration and acceptance or rejection. With a bank, I would count on rejection.
Basically, if you don't like the terms, find another bank or do without the card.