Using an applications window handle in a DLL?

Heya Folks!
After quite a while I start to program Windows again and as always I have a strange question. What I want to do:
I want a DLL to work as a plugin for the main application. I want to pass a handle to the applications main window to the DLL. Then i want the DLL to create sub windows and controls inside the main window.

Is that possible and is it allowed?. I seem to remember, that there were some restrictions on using handles across module borders.

Thanks a lot for any help!
Sonny

There is no restriction on using window handles across module boundaries.

Thank you VERY much!