There are Blueprint nodes for “Attach Actor to Actor” and “Detach From Actor”. Make sure the Mobility of your wheelchair is set to Movable first. That would be your first proof of concept and then you’d have to get into physics, sockets, animation blueprints, etc.
If you’re brand new to UE there will be a few more concepts to learn about communication between different actors in the world. This tutorial will get you started with the bare minimum and there are other ways to make it robust. Your box could detect overlap and then attach itself to the overlapping character. Or most likely your character will check for overlapping objects and then attach them.
You can also look at the VR Template project which has a whole system for Grabbable Objects.