Most one shot developers don't keep well for more then a day or so but as you suggest I replenish my Ilford/Harmon WT & CT working strength developers. For each new print session I mix up fresh developer and add about a third of oxidised old developer to make up to the total volume. As yet I've had no adverse effects, blacks are as deep as when using just fresh developer.

You may want to experiment and perhaps try a 50:50 mix.

Good luck,
Trevor.