In terms of this I think the best way to go about it is the following.

Use strength score to see if they can lift. Strength determines lifting capacity, so I think using this as an approximation works pretty well.
Obviously the enemy needs to be grappled. So getting the enemy grabbed is next.
Lastly I think you need to make as part of the attack another athletics check because you are trying to do something extra.

It would read

Hit a MFER with a MFER
When you have grappled an opponent you can choose to may replace one or more of your attacks with your grappled opponent as your weapon so long as you can lift their weight. As part of this attack make an athletics check opposed by the enemies acrobatics/athletics. If successful you may make an improvised weapon attack with your grappled enemy. If you hit you may deal damage equal to 2d6+Str to both the grappled target and the attack target. If you are targeting a wall, make your improvised attack against the grappled opponents AC.