Most software doesn't need pattern generation because the "Piano Roll" display serves that function. Usually the entire drum kit is on midi channel 10. A piano roll display of that channel will allow you to create patterns of any length you choose. The highlight, copy, and multiple paste function of the program will allow you to string your patterns together as you choose.
I creat midi drum patterns in one of 2 ways-
1. Play the basic pattern in via my keyboard and then edit from there.
2. Use a program called Jammer Pro to generate patterns and fills. Best I've heard in auto accompanyment software.