If Dhelmise and Pokemon like Jirachi was in a double battle and Jirachi Skill Swapped its abiility with Dhelmise would it power up steel type moves on top of STAB or would it not stack at all.
Yep. They get the additional 1.5x boost in addition to their STAB.
Source (Use Ctrl+F and look for Steelworker)
Hope I was able to help!