If you have CC3 pipeline:

Import the robot as an accessory.
Scale to the same size as the character.
pose the character to fit over the robot if needed. Pressing N opens the pose panel.
Click on Transfer Skin Weights in the Modify panel.
Select the "Other" category and auto rig it.

You're done! Don't ever conform it, as it will try to fit to the outside of the character!

You can also rig skeletons, replacement body parts, etc. this way.