Hi!
It is certainly possible... (I have done it) but... wouldnt you prefer to use something like:
http://www.cuyahoga-project.org/ (I havent tried it, but seems to be similiar to DNN and already has NHibernate support)
Personally... well I didn't like DNN internal API... of course... that was back... in version 2.x and 3.0... I felt that DNN was too intrusive (
I think there should be a way to build applications that allows you to test you module as if DNN didnt exists, and then easily embedd it in DNN or whatever portal framework you would like to use)... and solving problems inside it was like killing a fly with with an atomic bomb, but perhaps is better fit for what you want to build...
I think I like more the approach of ASP.NET 2.0 without DNN (on which reusable funtionality is achieved by components, and masterpages, instead of a surrounding intrusive framework that in .NET 1.x you had to wait ages to compile, and that sometimes (too often in my opinion) corrupted the VSCache)