Currently, it is a very arduous process to import rigs to Roblox that blend skinned meshes with conventional separated meshes bound directly to bones. You effectively have to split these imports up into separate halves and manually combine them.
Attempting to export a simple rig that combines these two concepts is a complete disaster. It creates strange extraneous bones and does not work at all, which means you have to do a lot of manual work if you want to create a rig that uses skinned meshes only partially. If you wanted to import animations on such a rig blending multiple meshes with both rigid parents and skinning information, you’re out of luck.
Here’s an image showing a naive attempt at importing an R15 format custom rig with a single skinned object. This feels like it should be possible, but the animation editor fails because the import has created a loop in the rig. Using the non-R15 format does not do any better or is incompatible with the idea of combining these.
I need to create these rigs because my game features custom alien creatures. These rigs use discrete un-skinned parts to fit into my art style; make it easier to create creature accessory parts like horns, whiskers, and swappable body part; enable ragdolls; and make content authoring a little easier since content is my main limiter - weight painting takes more time than the original modeling. However, I have certain custom characters that have wings and other membranous parts, which are nearly impossible to create believably with un-skinned parts, so I need to blend skinned and unskinned rigged parts.
It is currently a nightmare trying to create these rigs. These issues are currently holding back 3 new custom characters.