Let's say we have a gallon of beer to which we would normally add 1 oz corn sugar (which is about 46ppg and near-enough 100% fermentable). We need to add the same amount of fermentable to the fermented beer, as well as to the volume of the new wort. Let's say the priming wort is 1.040 and 70% fermentable.
46/16*.7 = we need about 2 gravity points per gallon, or 10 in total; we need a quart of 1.040 wort for 5 gallons. We need another twentieth of a quart for priming the primer, but let's round that to an ounce. We need 33oz of 1.040 wort to prime 5 gallons to the same level as 5oz of corn sugar.
Back of the envelope stuff....