Kamailio does support multidomain, check the 'domain' module.
The ability for the user from domain A to register to domain B is not usually allowed, because the user you create will be something like user@domainA.com
Domain module's doc is very self explanatory, give it a try :)
2009/7/9, zhangchao00001 zhangchao0000198@163.com:
Dear sir: I write to ask if the kamailio support multi-domain. for example, A UA belong to domain b get into domain a, could it work as well as in domain b? If so, how to configure it? Thank you.