Is it possible to add mods onto a modpack (for example RLCraft) and have them work?

So, I already have an RLCraft server. And everything is working fine, besides my one problem. I want to download an SCP mod and grave mod. So I added them to my mods folder in my curseforge rlcraft files but when I tried to use the stuff in game it wouldn't work. And if I tried to use forge instead of the rlcraft modpack would it cancel out rlcraft entirely? If so then is it possible to add mods onto the curseforge rlcraft software and have them work?