username input filter (where to insert)

[DC <dc dcarr org>]

Hi all,
I'm working integrating a new linux environment into our existing Novell 
one.  Unfourtunately many of our Novell usernames contain whitespace, a 
big no no according to my linux server.  Although probably not the most 
elegant solution, I want to insert a quick little filter into gdm that 
will take whatever the user typed as username (user string) and replace 
all spaces with underscores and then pass the username along.  This 
needs to take place before the username information is passed to PAM.  
My question is: Where is the best place (which function/sourcefile) to 
add my filter?  I've been looking at gdm_login.c but my C is not as good 
as that of the authors' of the file.  Thanks for your help,
-David Carr