There are single-cell chargers (such as the MAHA) that are supposedly able
to "rejuvenate" cells using a special cycle. I don't know whether they
actually do it, and you'd have to dismantle the pack to get at each cell.

Charging nicad and NiMH cells is based on current, not voltage.

I would suggest thoroughly "running down" the battery pack until the voltage
is 2.7V for several cycles. You have to be careful, because "deep" discharge
runs the risk of cell reversal.