Yes, it is feasible to mix DXLs that use different operating voltages on any ROBOTIS controllers. Needless to say it is going to need quite a bit of work from your part.
First read this post first for the general concepts involved in this issue.
Essentially you need to understand that, vis-a-vis the X3P connector/cable, there are 2 electrical circuits into those X3P cables: the VDD circuits which carry different voltages (5V, 7.5V or 11.1V) - which should be isolated from each other, and the DATA circuits which carry the SAME 5V TTL protocol for all DXLs.
You can also check out this post for some actual demonstration using a CM-904 and EXP-485 shield:
You can also use these X3P Hubs instead of using the 485-EXP Shield.
This post is also relevant if you are thinking making some custom X3P Power Cable to use with those X3P Hubs