base or upright? If its the base you can swap a passenger one on in about 5 minutes! If its the upright you can still use a passenger seat upright, but the angle adjuster knob will be on the wrong side, as will the tilt lever if its from/for a 3 door. But how often do you tilt the driver seat forward?
The other option is to get a passenger seat from a 5 door model and swap the fabric over, which is a little more tricky but still doable.
The last thing to worry about, is pre/post 90 seats are different so cannot be swapped over in this way! (well the bases defo are, but the uprights may be the same...)