Yeah, it’s easy. Import your breathing animation, built from the idle pose. Set it to “local space additive” inside UE4’s anim editor. Now, in the ref pose dropdown, select the Idle Pose anim you imported into UE4, and set the frame to 0.
This will configure the breath loop anim to extract additive data RELATIVE to the idle pose anim you specified (at frame 0, which I assume is the only frame if it’s a single-frame pose).
You can do this operation for every single additive anim individually; this way, you can build additive animations based on some known base pose and apply them that way. It also means that setting your breath loop to be additive based on the idle pose won’t have any adverse effects on OTHER additives you might have already set up based on T-Pose.