Although worm castings could be a good substrate for some mushrooms, I've never seen any info about them being used as a substrate for mushroom cultivation. Devon' s right- worm castings are lacking in "fiber".
Collecting spores from supermarket mushrooms is probably not going to work, either. My suggestion is to grow oyster mushrooms from stem butts on wood chips or almond agaricus from purchased spawn on leached cow manure. The numerous threads on this site about oyster cultivation and the books by Paul Stamets can guide you in the right direction. Good luck!
By the way, I've had BIG problems with earthworms in my mushroom projects over the years- earthworms love to eat mushroom spawn such as myceliated sawdust and grain! They can ruin an outdoor spawn run if they are present in your substrate (like
compost). But I love earthworms on my farm- I just keep them away from spawn runs, or they'll munch all the spawn!!