just make the .blb file in notepad.
eg... a 1x1x3 brick is
1 1 3
BRICK
Making custom bricks on the other hand is significantly more difficult. anything other than a cubic form requires you to map each individual vertex for the polygon. For example, a 1x1 round brick has 505 lines of script.
I heard several rumors that v9 would have a bit more support for this kind of thing, but i have yet to hear about any of these improvements.