Stop It!
WARNING! The above post and this entire thread are very dangerous!
You don't just substitute a payload just because it will fit! Any more than to ignore the amount of powder or bullet weight in a brass cartridge.
Steel airgun B-B's are extremely hard. The copper wash on them is very thin, just there to delay rusting. Steel shot, that is made for non-toxis shotshell loads, is much softer than airgun B-B's.
Then there's the wads. Steel shotshell loads have heavy, thick wads. This is to prevent the hard steel shot,(it's much harder than lead), from scraping the barrel, cutting groves in the barrel.
Ignore my warning, you will either wreck your barrel, more likely blow your gun and yourself to kingdom come! Any bystanders could get hurt as well!