I guess there is a better way, but I would try “Layered Blend Per Bone”. You can add as much layers/pins as you like and control the blending with the alpha value. So you could have a base pose like locomotion, and a slot for the wepons in each layer, and in the BP you can set INTs depending on the weapon which one of the layers to add to your base pose putting them in the alpha.
EDIT: I’m dump … create an enum with your weapons and you can use a blendPose OnYourWeponEnum after that in AnimBP - would be the easiest one 
hope you know what I mean, I’m not on PC right now.
You could also set the blendspace to use as a variable in your stateMachine - just in the blendspacePlayer there is a box to tick to make it variable 
so many possibilities yey