Non main-form to be mdi-form?

I'm working on an explorer-like application (tree + detail area)
that shall open another form as an mdi-form (I don't like thousands
of tool-windows being floating around on the desktop).

Is it possible to have a non main-form form being a mdi-form?

My workaround was using a form with some toolbars and a
scroll box inside, the scroll box being the parent for the
child forms. With some event handling (WM_SIZE, WM_MOVE) in the
child forms this worked, but unfortunately the child forms
didn't get the focus when clicking onto them with the mouse (the
tab key worked). What do I have to do to ensure proper mdi

Or is there another possibility to get rid of this?