I don't believe there is any weapon fusion that helps with wielding heavy weapons without the requisite strength, but you may be thinking of the Gunner Harness weapon accessory, or the Tactical Scaffold armor mod.
Both can be found in Armory.
You did not read the gunner harness correctly. It is actually much more powerful than you think.
The gunner harness does two separate things. The first thing is reducing the strength needed to use heavy weapons without penalty. This part is actually very badly written, but the intent seems fairly clear that with the level 3 version, you treat your strength as 12, and with the level 7 version, you treat strength as 14, for the sole purpose of determining whether you have a low strength penalty.
The second effect, which reduces penalties by 1 point for the level 3 harness, or 2 points for the level 7 version, reduces the penalties incurred for Full Attacking or attacking outside of your first range increment.
Ironically both of those solutions have lots of bulk and therefore require strength to carry.
There are other things that increase carrying capacity.
The gunner harness does not allow one handed wielding of heavy weapons.
The tactical scaffold does, but only assists with the strength requirements if you're using both hands.