posting a form from within a dialog box

I've been trying recently to have a small form pop up on a dialog.
I'm using asp.net and i find it very difficult to do this because the
dialogs are removed from the form#aspnetForm I can't seem to add the
dialog back into the form and I can't move the form to surround the
dialog box. What do you suggest and why was the dialog ui element
written in such a way that would make this nearly impossible