I think you can calculate the head space pressure for the temp you are going to store the keg. Then pump co2 into the keg at that pressure right after adding the sugar.
For example, according to Beersmith Mobile you need 28.8PSI head pressure (gauge pressure) at 70F to get 2.5 vols of CO2. Where 70F is the temp of the room where you will set the keg to carb up. So, add the correct amount of sugar. Then, pressurize the keg head space to 29PSI and put the keg in the closet for a few weeks.