Unless there is a leak - there is no reason to refill the lines with anything. It is a sealed system, and no refrigerant should be getting out - if there IS, you need to repair the leak, not just put more refrigerant in. Also - depending on what type of refrigerant it uses, you may not even be able to (Generally, only 134a is available to the general public). This is something you need the right tools to do, refrigerant pressure gauges, etc - so don't try it unless you know what you're doing and have the right tools.