Handling the dilog resize by myself

Hi all,
I have a very special case: a skin dilog (without thick frame, without title bar), movable from anywhere you klick ( OnNcHitTest returns HTCAPTION if the parent reterns HTCLIENT). Furthermore has rounded corners
CRgn rgnWindowRegion;
rgnWindowRegion.CreateRoundRectRgn(0,0,Rect.Width(),Rect.Height(),4,4);
SetWindowRgn(rgnWindowRegion,TRUE);
This dilog is not resizable, but it must be one if the user clicks on a special button, and drags the mouse with the left button down. Has anyone an idea how can I do this? May be a similar example?
For any suggestion thanks in advance.

Regards,
Emanuil