Hi Kindermantltd,
Draw your custom shape as a closed polyline, pick the shape and right click,pick convert to and then pick profile definition, pick insertion point and then hit enter, name the new profile definition and then delete the shape. Use the opening tool to put an opening in your wall, pick the opening and go to the properties pallet, under shape at the top pick custom, below that pick profile and change it to your custom shape.
That should do it.
Let me know.

Hey Newbie,
The only way that I know is to do as follows:
Lets say that you have a 2'x 2' retangle that you want to make your hole from, and it is 24" above the bottom of your wall. Draw the rectangle on your wall, on the design tool pallet select the opening tool, pick the wall that you want the opening in, then go to the properties tool bar and set the sill height of the opening to the height of the bottom of the rectangle that you drew, eg 24" above the bottom of the wall, lastly, pick on the hole that you created and using the dynamic grips, size the hole to our rectangles profile.

Hope that helps