Are you running the latest version? I don't have a BC15 to test on any longer, but my copy of ARC15Lite allows for setting up custom band plans. I downloaded the information from the RR database and when I select "Motorola 800 Mhz Type II Custom" an "Edit Custom Band Plan" button appears. When I click on that I get a new window that has a custom band plan with upper and lower frequencies, offsets, and steps. I can't verify if it's the correct information, but it's there.
I have version 4.000 build 1.