Hello Aman,
or you could use a script on that attribute in the standard "createABAP user" task that checks the repository and sets the parameter when this repository is detected and leaves it blank, when it's another.
I try to keep away from maintaining different hook tasks when I can set values through small scripts. ^^
Regards,
Steffi.
PS: My spidey sense is tingeling... wasn't there a similar question some time ago? I feel like I already wrote something like that. 