The RAM Frame - Criteria/General/Wall Elements/ "Include Rigid Link at Fixed Beam-to-Wall Locations" option will extend the beam into the wall when there is a lateral beam that is (approximately) co-planar with a lateral wall and that beam has fixed ends. It sounds like just what you want. Other beams will not be affected by this criteria unless they pass those tests.
Regretfully assigning rigid end zones is not something you can do member-by-member at this time. The rigid offsets in the Modeler are for shifting a beam slightly off the supporting column centroid primarily. I'm not too surprised they won't work for your task.
Regretfully assigning rigid end zones is not something you can do member-by-member at this time. The rigid offsets in the Modeler are for shifting a beam slightly off the supporting column centroid primarily. I'm not too surprised they won't work for your task.