Walkers are just closed loops of passengers randomly walking through that path, you can place them anywhere.
However, since there can only be one walker path for each gate, it's probably best to keep the path in the general area around the gate they are related to. Mostly from an organization point of view, since it might be confusing for you when editing a path related to some gate, with waypoints placed on the other side of the airport, even if it's still technically possible.
