Active Directory from extranet to intranet domain... Impersonation

Crap, created a application page that reads Active Directory on a Intranet.
But the infrastructure department has deployed it on a sharepoint on the Extranet domain.

Result, could'nt query the additional information in the Active Directory of the users on the intranet.
Somebody told me to use impersonation, but have no experience with this. If someone can help me... 
don't hesitate...

If I found a solution, I will keep you informed...