Add a field when entering the field

[Mastacheata]

You didn't ask any question yer, but I guess you want to know how to realize this.

Do you want this to happen on the fly? (e.g. the form changes input field with the information from the select box)

Or do you simply want to add both inputs to one string for processing as the domain field in the following scripts?

Both need to change the user_add.php since there is no Template system for the gui where you could place the javascript needed for variant A in ispcp, at least as far as I know.
Variant B would even with template need you to change the php because you have to change the content of the two submitted variables.

I don't think either variant is a good idea unless you know what you are doing.
Any changes made to the Source Code Documents will probably be lost with the next update and you bring in possibly new security leaks.