It's a simple geomerge, unless you want to merge their animations as well.
To do a simple geomerge, just delete the footman's sword, take froastmourne and add it to him, afterwards select froastmourne and attach it to the bone that the footman uses for the sword.
I suggest you to use oinkerwinkle's geosetmerger.
I don't know how advanced is your knowledge in this area, but if you need a further description, look around wc3campaign's MDL Editing tutorials.