Should be able to balance while its charging, all hooked up together. If you balance while its off the charger, any balancer will just discharge the highest voltage cells down to equal the lowest voltage cell. This may cost you capacity depending on how much diff in voltage there is.

Usually this type balancer can do both, act as a standalone balancer (sometimes called passive mode) and an "active" balancer balancing cells while batt is charging. The active mode will give you a batt charged to its highest capacity. Should be just a matter of reading instructions and making the connections.