If you get the harnesses that clip into eye-bolts then it's easy.
Just replace the standard seatbelt bolts on the centre consol and bottom of the front doors with the eye-bolts, clip in the lapstrap of the harness and run the sholder straps through the gap between the back and the base of the rear seats and clip them into the D-plates that the rear seatbelts attach too in the rear floor.
This way you can retain the standard seatbelts and just clip the harnesses in when needed.