Ni batteries are balanced by putting them on a trickle charge. The classic NiCd charge routine of 0.1C for up to 24Hr would balance the pack by driving every cell into overcharge, where excess energy is dissipated as heat, and nothing horrific happens for charging like this.
NiMH can vary in its ability to be overcharged for a long time, and their capability to burn off the overcharge without building excessive pressure internally, so 0.1C is not always safe. Especially in low self discharge NiMH cells. If your cell manufacturer gives a trickle charge recommendation, follow that. If not, personally, I'd go for 0.05C and let it sit for 25-30 hours, while making sure it's not getting too warm while doing such.