Adding weapons to poses/animations?

I'm actually quite the beginner at modding and I was following Mr. Cellophane's Animation Tutorial for 3DMax. So far so good. Made a few poses, everything went pretty good thanks to his tutorial.

Then after a few hours i was asking myself how it is possible to add some weapons/items like this found in OSRPose:


I've tried importing a standard mesh into it but it didn't show up. Maybe there is a guide somewhere or someones know how to do this/can give some hints/tipps?

Thanks!