Within the path editor, you really want to change the relative position of strokes with the regard to each other. If you want to move the whole path, use the Move tool in "Path" mode. But remember that if you "link" things (Layers, Paths, Channels...) then everything linked when you apply a transform to one of them applies the same transform to the rest, so maybe you don't need to merge the paths, but just need to link them.
If you want to change visually close but distinct "strokes" into a real single stroke, the "Join strokes" function of my
ofn-path-edits script maybe what you need.