I’ve been attempting to make a new AimOffset with custom animations exported from Maya and I keep getting this error when I attempt to drag them into the blend space editor: You cannot mix animations with different additive types in BlendSpace.
And I’m stumped. Not sure exactly what the issue is? Is it animation layer related in Maya?
Nope, it’s not Maya-related at all. In UE4, when you define an animation as additive, there are two different types of additive that you can choose from, namely Local Space and Mesh Space. For a blendspace to work correctly, all of the poses or animations it contains need to have the same additive space setting. FYI, in the case of aim offsets, it should be Mesh Space.
For detailed info on the difference between local and mesh space, see here: LINK.
Good luck!
Link broken 
Anywhere where I can find an explanation of the difference between local and mesh space and when to use Apply Additive and Apply Mesh Space Additive?
Thanks
https://docs.unrealengine.com/latest/INT/Engine/Animation/AnimHowTo/AimOffset/01_CreatingAimPoses/index.html
↑Document of creating Aim Offset.
This may help you.
Late answer but for anyone wondering, for aim offset, select all your aim poses, bulk edit via property matrix, under additive setting set anim type to mesh space, base pose to selected animation frame, and select your idle base pose animation as the base pose animation. Hope this helps!