How can I weld a part to another Instance?

How can I weld a part to another instance? Like I want to weld a part to an instance that is not a part. I've tried it with a WeldConstraint but it says that it can only attach to parts. The specific instance I want my part to attach with is a Bone.

Here is an Image of it in Explorer. I want a tool's handle to be welded to my Arm Rig's Palm. But the Palm is a Bone Instance, so I am having trouble attaching the tool's handle to it.

Please feel free to post your opinion. Any help would be appreciated.