Cutting Or erasing. bitmap

Hi. 

Im trying to get a chartacter to enter a doorway but to do that i need to erase the part that is going behind the door so that it looks like hes gone in. Ive converted it to frame by frame and made them all bitmaps however i cant erase it and when i do modify>Trace Bitmap It gets rid of alot of the detail on the characters shirt. Is there another way i should be doing this.

Thanks