There are a few ways to go about it, but what I would suggest is making the animation of the character in blender, then using a NOTIFICATION on the animation in unreal to spawn the objects at the appropriate times. For example, set an notification event marker in the animation so at the half way mark, two katanas are spawned and attached to sockets on the characters hands.
If you’re not familiar with notification events and sockets:
Of course other people might have a better way of going about it, this is just how I would handle it, and it works pretty well for me.