Alright. So. You want to use the "Decimation Master" tool, though make sure you have an iteration of your final sculpt saved off first.
This tool decreases the polycount of a subtool, while keeping the shape as best it can. Don't go too low, but you should be able to decrease everything to a few thousand polys each instead of millions. Use those to import into blender for retopo, but still bake from the pieces in the detailed final sculpt file.
Zbrush has retopo tools, but they're kind of questionable at best.
If you need more advice, this forum can be really helpful but sometimes the art sections are a little quiet.
Try asking at Polycount for advice that's program specific, there's just more traffic that actually uses niche tools for game artists in their workflow.
It's a great resource in general and for networking as a 3D artist, too.
Good luck, hope this can help.