I added 6" Peerless casters to my Harbor Freight shop crane. I attached them to leg extensions by welding them on, two beads on each caster. One bead is on the outside end of the leg, and another is on the opposite side of the caster's mounting plate.
The ugly color and irregularities that you see on the weld bead are from brownish rust preventative.
IMPORTANT: If you try to do a similar thing (weld swivel casters), MAKE SURE TO USE A WET RAG near the weld area, to keep bearings cool and prevent seizing. Doing that was a major pain, but it helped to keep bearings in their original, smooth condition.
I used blue marine grease, applied with a grease gun to the grease fittings, to prevent rust on bearings. I used spray on CorrosionX HD to protect weld areas from rust. I will keep this crane outside. (the hydraulic cylinder is kept inside). In a year or so, I will report if rust is becoming a major problem.
The casters were purchased for $10 from this highly recommended eBay seller.