I use 100 % Promix in my outdoor pots. I have about 25 of them going. This is my 3rd growing season. The first year, I didn't know that Promix had no organic matter/nutrients, so I didn't add anything to it, and the plants grew great! The 2nd year I mixed in 1/3 compost, and got pepper maggots, which I assume came in with the compost, and the plants didn't grow any better than in pure promix. This year, I'm back to 100 % promix. The only thing I add is TomatoTone (in a generous quantity, about 1.25 x the recommended dose) for the nutrients, mixed well into the pot at all height levels so the growing roots pick it up over time.
I don't see why putting in some topsoil would be a problem. Obviously, it's heavy stuff, and if you add too much, it will have a negative impact on drainage. But in small quantities, why not.