I've done this in a number of ways.
Pop up the DLL UI as a form, it'll appear to the user to be a form within the main App.
Pop up the DLL UI as a borderless form nested within a user Control in the main App.
Implement user Controls within the DLL and use those as your UI.
I've done this with WinForms with absolutely no problems whatsoever.
Actually, now that I think about it, I have some sample code that might solve your problem,
Here[
^]
If you have any specific issues feel free to come back to me.
Good Luck.
-Richard