If you have crushed lava rock and grit (you could likely omit the sand) that's a great mix. Basically any soil you can mix up that will dry out in 2-3 days is all you need, you could even use shredded car tires if need be (Singapore)! There's nothing magical about Napa, it's just easily available here and does what I want, Shultz Aquatic Plant Soil does what I want too but it costs way more than the Napa.
I'd just poop with glee if I could get some small crushed lava rock or small pumice in my area! That's what the Napa or Shultz APS is being used as a replacement for in my mix because we don't have those things available here.
As far as returning your Lithops, even in the "store soil" you'll probably keep them alive longer than the store will! lol! But if you've got crushed lava rock in small pieces, you're already all set! Earlier you said pumice and grit but that would be really dry since water only sits on the surface of these two things and doesn't soak into the pores. But pure lava rock or lava & grit should be awesome to pot them in. You might even need to add a wee bit of peat to force it to stay wet for 2-3 days. The best thing to do is run some empty pot experiments and see how long a few different mixes using what you have onhand take to dry out after being watered. The mix which is dry clear through but not compacted in 2-3 days is the one to start with and grow from there. Whatever gives you that result, no matter what materials it is (so long as it's non-toxic), is a good starting point. Some succulents you'll find like a moister mix and some a drier but 2-3 days generally allows them all to soak up enough water but doesn't stay wet long enough to induce rot in healthy roots. Mesembs like Lithops are some of the worst "rotters" in the MN climate. If the soil dries faster than a couple days they might not get a decent drink.