This is the name that ChemDraw give me if I put an ester on one of the termini of citric acid:
2-hydroxy-2-(2-methoxy-2-oxoethyl)succinic acid
and the systematic name for citric acid:
2-hydroxypropane-1,2,3-tricarboxylic acid
So it looks like you name the ester sidechain separately and tack it on to the name for the diacid.