I want to customise my contact form - New Donor
The language of the question is a bit unclear, but I think what you want is a token that takes just the first letter of the first name.  You don't need to collect it separately to do so.

If this is what you're looking for, please leave a comment to this answer saying so.  I have an extension that adds a "middle initial" token, I could quickly adapt it for your situation.