Let's say the exchange grants you 0.0005% of the trade volume as credit for every maker order that get's taken. Can Zorro simulate this? It's technically a negative commission but Zorro interprets negative values for the Commission variable as a percentage based commission, not a credit.

Just use enterTrade.

0.0005% is probably irrelevant for backtests. If you want still simulate it, add it to the Balance variable after entering the trade.

I realized you can set a negative Spread which will be added to the Balance instead of subtracted. Probably a little more intuitive than manually setting the Balance but similar.

I actually wrote about this on my blog:
https://www.vitaltrades.com/2020/07/14/adding-exact-fee-and-credit-calculations-to-zorro/

So basically, you can set Commission to 0 for your trading asset, and then use a dummy asset to add complex commissions calculations in the form of standalone trades with profits/losses.