How to create "landing" waypoints?

Manual says that in order to get the synthetic runway you have to have the waypoint pointed to a landing strip.

Lets say I need to divert to a different airport (so not pre-entered in the FP), which I had the coordinates to enter, is there a way to do it so I can get the synthetic runway on a 0 visibility landing situation?

No, there is no way. Razbam just did not implemented this function correct.

Only way to activate a synthetic ruwnay is, to set the landing point in the ME before flight.