Only ways to get more slots is to equip a talisman (up to 3 slots) or a weapon with slots (up to 3 slots). Maybe solving your problem with capturing would be more profitable? What is the issue exactly?
It's just that I can never seem to tell when a monster is weak enough for me to trap it. I can see that it's drooling or limping, but each time I try to capture it it manages to break free. Then I try weakening it some more but I end up accidentally killing it.