I started having to do this myself actually. I normally would get a bunch of Happy Frog, and then use that as the soil, and add the Nutrients I use to that, but this season was a rocky start, and we didn't have the cash for 300 dollars worth of soil, so, I decided experimentation time was a good idea.
Mind you, the reason I need so much soil, is that, first I star my seedlings in solo cups, which isn't much soil really, but then once they are going outside, I have 20+ pots, most of which are 10 - 15 gallons each. Clones I just stick in a 5 gallon, but seedlings get huge out here, so, I need a larger pot to work with. Makes them harder to knock over in the wind basically.
Normally, when my season is done, I take branch trimmers, and cut the main stalk off, and toss that in a bon fire pit, and then the soil, and the root ball, all would get chopped up nicely, and then, I'd take a big metal rake, and rake it back and forth to form a "base layer" and then add in ashes from the wood stove, and mix it with that top layer of soil, and then my pots go right on top of that. This season though, I ended up taking a few pots full, and making a big pile in the front of the garden, and then, I'd go out there, and add in beneficial microbes (Recharge) and stale Mountain Dew / Energy Drinks, and whatever else that was food based.
The in Law uses soil again sometimes too, by simply taking it, putting it on a tarp, and then covering it up with the other side of the tarp, and then, a few weeks before using it, he adds in a few bags of bat poo, and mixes it up.
When I started bringing seedlings outside, I didn't have bat shit, so I took the main root ball out, chopped up the rest, added in decomposing leaves, pop, sugary stuff, and then empty nutrient bottles, I took those and filled them with water from the hose, shook them up, and dumped that on top of the soil, and then started adding in normal compostable paper bags and whatnot to it, and then left it in the Sun for a while. When it was time to fill all those pots, I grabbed some of the previous soil mixed with pot ash, and added it to my pile, and then just shoveled it into the pots until they were full. After the plants were in the pots, I watched each plant to see how they were taking it (looking specifically for nutrient deficiency issues, or nutrient burn and so on) and of course watching for pests.
After about 2 weeks, I started giving small doses of nutrients to my plants to see how that went. When I didn't notice any issues, I continued, and eventually gave them full doses of it, but I wanted to make sure that whatever was left in the soil wasn't going to be an issue. My lack of experience with doing this, made me a little light handed on nutrients at first, because I didn't trust my skills. That was really the issue though, because looking back, I could have totally given them full doses of nutrients, without any real issues.
The main thing I think you should check, other than of course, looking for bugs that were in the soil, is that whatever plants you put in that used soil, may take nutrition a little different than some others, so, watching for burn is totally recommended.
Now, as my season finished, I had good results honestly, so I plan on doing this again. I don't have every pot out there emptied and in my "pile" yet, but I've added a few pots to it worth of soil on days where the weather was nicer. I'd say get some type of "Recharge" and add in sugary pop, or just "Sugar Daddy" from TechnaFlora (The brand I use for my plants) it's basically molasses and pot ash, and feeds the microbes.
I've since added a few other things to that pile, like more paper bags, cookie crumbs from cookies that got stale (even if a critter digs up the pile for the cookies, they might just pee and poop on it, adding to the nitrogen levels) and I also added a Cheese that got moldy (busted it up into pieces, dug out the pile, put it in the middle, and on top, and covered it back up) and even a beer can I had that... LOL, I got a 12 pack for my Birthday in November, and one can was only half full, so I wasn't going to drink it, but, it's beer... I dumped that out in my pile too.
I'm still learning obviously, but so far, my results have been really good. When you first start out, experiment.
Also, if you DO end up getting bugs that aren't friendly, you can use a tea kettle to heat up water until it's almost boiling, and then, dump the water on the soil in a bucket or something, and it'll kill pretty much any bug and it's eggs, fast. Only real issue there is it might kill microbes too, so, if you can, do that before you add in some type of microbes.
