Recharge a dry aquifer?
Fossil groundwater is water that has been trapped below the surface for several thousands of years. It is considered non-renewable as natural recharge on human timescales is practically zero. This is because the permeability/transmissivity of the overlying strata is very low.

However, on geological timescales the aquifer will recharge as pumping the water out of the aquifer decreases the hydraulic head, and therefore increases the gradient over the aquitards sealing the aquifer. Therefore the recharge of the aquifer actually speeds up by withdrawing water, although probably not in amounts humans can profit from.

Since you're asking about techniques to recharge, techniques similar to those used for shallower aquifers could be applied. In the Netherlands regeneration of groundwater is performed to decrease the stationary drawdown of the groundwater by the pumping which leads to damages for farmers. This is often performed by pumping surface water into the aquifer at some distance so the aquifer is recharges artificially and the groundwater table rises.

On the other hand, recharge of these fossil water aquifers in arid areas such as the subtropics is probably infeasible, even though it would be technically possible.