Add a field when entering the field

[mafia]

Hello,

So I appeal to you because I have a little information,
Let me explain:

I add a scrolling list with several extensions (.net / .com / .com etc. with subdomain example. C.la .. etc) at the customer's order at the time of entering the field:
So to summarize I do this: in the current field are among the field without the extension and then a drop-down list or he chooses all extensions allowed.

I hope I have made clear
Thank you in advance
RC7