How can I couple 50Hz/60hz mains interference to a signal cable in practice?
How can I couple a significant 50Hz magnetic inference to a cable in
  practice? Is there an easy practical way?


I don't know about an "easy" way but you could use a three winding transformer with windings in series with signal and return wire (shield/screen) and the third winding excited with a 50 Hz voltage: -



You can "inject" at the source end or the receiver end to suit the configuration of your cables. The circuit above creates a common-mode voltage and that is what I believe you are looking for.

The signal source (50 Hz) needn't be connected to earth - it can be isolated but it was convenient to show it this way. The signal source could be the low voltage output from a seperate mains powered transformer.