How do I create an object footprint resource?

I have Simdougnut's Freezer Bunny Ice Cream stuff, and his sellable pints and quarts of Ice cream are missing footprint resources. He hasn't updated the set in like 4 years, so I'm thinking it's not gonna happen. I tried to contact him, but have yet to hear back (I also hate asking a creator to update something, it just seems rude). So, in lieu of all that, I thought I would just add the footprint resource myself. However, I can't seem to figure out how. The objects both have meshes, and I tried exporting and then importing them, to no avail.  In fact, S4S said that the meshes were missing a vertex (I dunno what that means).  So I was hoping someone would be kind enough to point me to a tutorial on this or maybe walk me through it. Thanks for reading, and I hope someone can help.