Koji is one of the most wonderful things. It makes amazing things from shiokoji, to miso, sake, to amazake. Just fantastic!
If the miso isn't sterilised, it may be possible to get some spores. I think it will be a challenge.
Koji spores at a warm temperature whereas miso is fermented at quite a low temperature. There is also a lot of salt in it to prevent the koji from sporing. However, if there are air pockets in the miso when it is stored, the koji does tend to grow there.
Actually, the more I think about it, the more possible it may be.
If I was trying this, I would try spreading some miso on a plate, maybe about 1/4 inch thick, keep it at room temperature for a few days and see what kind of mold grows on it. Koji starts white and then goes green when it's ready to harvest for spores. If your miso does that, then scrape off the green mold, dry it, then attempt to
culture some koji rice.
The easiest way I found to get koji spores is to buy koji rice at my
local Japanese food shop. This is usually in the frozen section. I didn't know the first time that I needed to keep it cold, and I put it in the cupboard. When I went to use it, it had turned green.