Sorry, I think I may have misread your question.
Are you creating sprites with custom box colliders on them? What exactly goes wrong?
I created a 2dtk SSB object, and then created a separate 2dtk sprite, in the hierarchy.
For the sprite, I set a box collider (and also tried using a mesh collider), duplicated it 4-5 times and repositioned the clones.
Once everything was set up, I parented them to the SSB object.
My SSB object was set to generate a collider, but when I committed it, no collider was created for it.
Additionally, when I edited it again, the sprites sometimes lost their colliders.
All I'm trying to do is batch together a lot of sprites to use for platforms/walls, that way I can just perform collision detection against that single batch collider, instead of against lots of individual colliders (or any other collision detection means).
Regards,
Justin