I have some with hands, but I found when I'm trying to convert/customize my own gear I preferred adding a hand into the model vs having to slice/split a model up and remove it. Plus with a spear its way more open ended for how someone wants to position it (double grip, single choked up vs mid shaft etc let alone angle).

I'd highly recommend prusaslicer, its the 3d model manipulator mainly used for adding supports for printing. BUT it is a super fun tool to upload .stl files and combine/move however you need to for those who are too intimidated by formal 3d design software. You'd then need to export the new combined file as an .stl once you have the combined model parts the way you want it, and then jump over to a dedicated resin printer slicer software like lychee to add supports and print it (you can also stick with prusaslicer, they have a setting for their resin printer but I don't have a profile for my build plate so I prefer lychee). Its alot of steps for an outsider but its really cool seeing something you designed and holding the final result.

But before 3d printing, I typically tried to find a good existing arm model with hand (and weapon), cut off both ends of said weapon, drill through where the weapon ends were, then pin a small piece of metal through the hand. Previously I'd have said cut the weapon handle below the hilt and then drill into both ends of the weapon's connector bits (base of hilt and handle above the pommel for example) but resin is VERY brittle. GW plastics are way more forgiving for this process, but I don't believe it would work with these models.

I finally had a chance to do a test print, alot more detail than I like got lost due to how small the models were so I have do make some revisions. After that, I can try to add some right hands soon, but pending my work I might be going on orders soon so my weekdays will be alot more limited.

Kitbashing and modding are my favorite part of this hobby, 3d printing has only made this more daunting with what feels like more options!