Sterilization seems like the wrong concept for a worm bin. We need the microbes.
I sometimes throw a little dirt or sand into the bin, definitely NOT sterilized.
But I don't actually any think dirt or sand is necessary. For years I had composting worms before anybody told me they needed grit, so I never added any. The worms still did fine.
mbetts

I guess my main concern would be parasites from feral cats, etc. What do you think?

Thanks!

worms aren't bothered by things that get after you and cats .... but you don't need to worry about grit in a well functioning worm bin - I haven't added any sand to my large years-old bin ever, tho' it does get a bit of soil from plant roots

if you really want to add grit, dry and powder some egg shells and add a bit once in a while, adds calcium as well for insuring neutral pH of the finished castings

Bill