Actually, disregard that. You will have to delete the IK_ prefixed top-level bones (displayed on the same level as the ‘root’ bone); but the animations should retain skeletal compatibility as long as you don’t delete any other bones or add any in the middle of a chain.
Note that Blender’s coordinate space is different than the skeleton’s, so your animations may not actually be usable. Some playing around with the orientation of your skeleton, blender’s export settings and the animation asset’s import settings may be required to resolve that.
If you can get that resolved, I would greatly appreciate if you would post the settings necessary to ensure coordinate-space compatibility. If not (as I fear may be the case), you will need to import your own skeleton.
Tangentially, there is a very slim possibility that you could retarget meshes based on the default Epic skeleton to your own imported version of the same skeleton, provided the bone structure and naming are the same. I have no idea how to do this, but it seems possible, what with talk of Skeleton Retargeting. Pure conjecture, mind you.