I've never mineralized soil, just chosen very carefully to get plain soil that doesn't have "3 months of fertilizer!", worm castings or poultry whatever (chicken s@!t) in it. Admittedly this is getting harder and harder. I'm just restarting small tanks that have been ignored the past couple of years and in the end got plain top soil from Home Depot and added plain peat moss to it and used that. I should have just given up on the big box stores and gone to a nursery to start with. That's where I ended up having to go to get plain peat that didn't have anything added to IT. Am getting more tanin color in the water than ever before, which I suspect means I got carried away with the peat, but so far so good other than that.
Haven't done it, but have seen reports from others that yes, they've added soil to an established tank either by wrapping in wax paper and putting under existing substrate, then slashing the paper with a knife after the packs are in place or by freezing and putting under. Freezing sounds better than having paper wrapping disintegrating under the substrate to me, but you'd have to put frozen soil in a little at a time to keep the temp from affecting established plants and fish, I'd think.
The only negative for me to the soil based substrate is the mess when moving things around. The times I've gotten tired of it and set up a tank w/o soil though, I get tired of having to add this and that and tweak this and that fert-wise. My 2 wpg soil tanks never have algae problems except diatoms, which ottos take care of, and the only non-soil tank I had for any length of time went through several kinds. So it's a trade off and everyone decides for themselves I guess.
