Allow subdomain

[santerref]

Hello,

I have a little problem and I'm sure that there is a solution for it. I look a bit on the forum before asking using search but I do not really find my answer so here it is :

Is is possible to allow ispcp to register new domain that are sub-domain of a domain that already figure in the database...

Like I have a domain registered that is domain.com. And now I say to my client : We provide free sub-domain like YOU.domain.com but when they try to register a sub-domain, ispcp says "Domain with that name already exists on the system!". So how can I fix this problem?