I would expect them to hybridize, also a path you would put outside would stay genetically exactly the same patch, this is due to the rhizomes staying in the ground when the grass dies back. It would take a long time - if it was possible at all - to get anywhere near a natural form, because against common believe it will take the plant a long time to really grow big and healthy if started from seed. The hybrid seeds wpuld be scattered over large areas and the resulting plants would be hard to tell from the native plants.
If there is a wild form around I would simple generously dig a patch up, put it in the sink amd water it to flush all dirt. This leaves you with all kinds of rhizomes roots ... Now you would have to identify the phalaris rhizome and carefully seperate pieces as large as possible. You can then plant those rhizomes in a pot or tray or outside and they will quickly recover and make a nice adult natural form Phalaris plant. Here I simply assumed you want to have a natural Phalaris (?)