change the UserAgent of the visitors of my site

Visitors come to my site through a pop-up launched with a given User Agent (i.e. Vomba). I want to be able to change that user agent. say pop-up shows [login to view URL] with user agent "vomba". Then [login to view URL] shows [login to view URL] in an iFrame. I want the user agent string passed to [login to view URL] to be different from the one in [login to view URL] : this user agent should be taken from a list inside the script or in a separate file. Please detail what language you would code in. I must be able to read and modify it.