I'm no expert here as I'm a beginner with multicam, but I've been doing a multicam concert with 5 cameras and bunch of B roll shots of audience. As the Multicam timeline is all on track V1, I've just been adding the B roll shots whenever I want them on V2. Works fine and doesn't bother the Multicam . I've even moved some of the shots from V1 to V2 and then changed the V1 shots and used the other shot on V2 in a window or for a double exposure.
You might want to lock the multicam on V1 & A1 when you bring in the B roll just to make sure you don't break it.