R15 rig for Blender with IK/FK switching?

Easiest start is the official Roblox Blender plugin, it pulls your avatar in already rigged. That rig is plain FK though. For IK/FK switching I built my own on top: IK constraints on the forearm and shin bones, then a custom property that drives the constraint influence 0 to 1, so one slider flips between modes. Bake the action before export or the constraints don't come across. There are a couple of community R15 rigs on the devforum that already have this set up if you'd rather not build it.