5.13 Can I manually build a connection when booking online? explains how to use AT/BS fares to test whether a custom connection is valid, as well as the rules that apply for non-standard routings. Not knowing anything about the procedure for actually booking a Buddy Pass itinerary (is it even done online?), I can't say whether it will be possible to do so yourself. Generally speaking, the web site no longer allows booking of WGA custom routings.