Kegs and glasses

I have been brewing different kinds of beer the last few months, notably Axemans' delite, Cider and Slayers' respite. When I started brewing, I had no Calquat tree planted and therefore I stored by beer in glasses to be used later. Now I do have Calquat kegs at my disposal and I use those to store 4 glasses worth of beer at once. I have found ways to pour beer from a barrel into a keg or into glasses and ways to pour beer from a keg into empty glasses but I cannot find out how to do the reverse; filling kegs with beer I have in glasses. I feel stupid that I haven't found a way until this day. The task seems easy enough, but I can't get the beer back into the keg.