Once you import each of the poses you need into Unreal Engine, you will need to set some specific properties on them to make them compatible with Aim Offsets. Under the Additive Settings category, make sure to set the following:
Aim Offsets only accept animations with an Additive Aim Type of Mesh Space.
Mesh space is a bit of a different approach for extracting delta of additive animation, in that it is only used in certain instances such as Aim Offset assets. Mesh space uses the mesh's bounding box as its base for rotation, allowing rotations to move in the same direction regardless of the orientation of chain of the bone in the Skeletal Mesh. Consider a character that was leaning to the side, but needing to aim a pistol upwards. If that upward aiming motion was in Local space, then the aim would rotate outward, in the direction of the lean.
Base Idle Pose
Leaning with Upward Aim (Local Space)
Leaning with Upward Aim (Mesh Space)
This is solved with Mesh space rotation. Since rotation is based on the Skeletal Mesh's box, the orientation of that rotation remains constant no matter what the character is doing. So even though the character might be leaning sideways, the upward aim would still rotate in an upward direction.
Mesh space is set as a property of an Animation Sequence. Under the Additive Settings category, you will see the Additive Anim Type property. Setting this to Mesh Space will utilize the Mesh coordinate space as described. This is a fairly expensive blending operation, and should generally only be used by certain types of Blend Spaces where you know you need the animation to move in a constant direction, such as in the case of an Aim Offset.