As of today (Dec. 14, 1995) JANUS does not yet offer the possibility to transfer labels (i.e. Label objects) from one architecture to another. Once this feature will be implemented, you will just have to load both architectures and call the label-transferring function.
For now you have the choice between a) writing the path found by the existing recognizer (puts method of the Path and Label objects) and applying some mapping after reading the path, and b) applying some mapping before writing the path to file.