What you are saying *sounds* good...I have been wondering the same thing. I have installed heaters in yachts by simply adding the coolant lines and more coolant, so I would *assume* the reverse is the same idea. This is also a question I have had.
Anytime you introduce air into the cooling system, you should burp it out. Normally, you'd run the engine with the heat on high and the radiator cap off for a bit and as things naturally cycle through, the air will be forced out.
Hope this helps a bit.