I have an aftermarket fuse panel too. You can add circuits by just connecting wires to the unused spade connectors and installing a fuse. (until you run out of slots) You don't have to take the panel off the firewall.
However, I bet that my panel is different from yours as there are lots to choose from. Can you provide more details about your panel? Make and model number would help. Who did you buy it from? A picture would be super.
