Do you need to add priming sugar when moving beer to a keg? Will the CO2 from the system be enough to carbonate it?
When you want to carbonate in the keg, you have both options: priming and force carbonation.
Priming is much as you would do it in the bottle. Just figure out how much carbonation you already have, how much you want and add the appropriate amount of priming sugar. The downside is that the secondary fermentation initiated by priming can take weeks to achieve proper results. Here's a good introduction to priming in kegs.
Force carbonation relies on the solubility of CO2 gas you're already using to dispense to achieve carbonation. Typically the beer would be transferred to the keg, chilled, then pressure applied. This method is very precise because the solubility of CO2 at any given temperature/pressure combination is well-known. Downside is that you pay for the CO2 you're injecting, rather than using (dirt-cheap) sugar to create it. Also, unless you agitate the beer while applying pressure, it can take many days for the gas to seep into the beer. This is why many brewers choose to use high pressure at cold temperatures for short periods of time while agitating the keg to speed up the process. This chart will give you an idea of the temperature/pressure relationship on the solubility of CO2