Sure, to convert a rigid caster to a swivel caster all you have to do is:
- figure out how to put some sort of swivel onto the existing rigid caster.
Which means figuring out and procuring a thrust bearing with the necessary thrust load capacity and also the necessary radial load capability (because there will be some sort of 'shear' plane load on the thrust bearing from moving it around or rolling it across the floor and any irregularities). After all the figuring and procuring, then all you have to do is the fabricating.
Easy.
By the time you do all the figuring and procuring and fabricating, even if your time is worth almost nothing, I'll bet that those $100 per caster RWM ones don't look all that bad now.
Or buy two of the $20 rigid casters from SurplusCenter and two swivel ones. Done.