There are two ways to wire it in. If you wire it as part of the main pack, you will need to get a voltage shunt from @Inwo for the harness and to charge the original 6 and the +1, you’ll have to disconnect the 7th battery connect the battery cables back to each other to complete the circuit for the on board charger.
Or. You can put #7 after the main contactor rather than part of the original pack. In that instance, it won’t be seen by the harness so the controller won’t try to error and shut down on over voltage so no shunt needed and because the contractor is open during charging, the original pack of 6 is isolated as #7. So no battery cables to mess with. Just turn on a 12v charger connected to #7.
Here’s a thread on the latter method that Dave and I tested out a few years ago. I ran that setup for a year or more before I snapped the end off a half shaft and parked that cart for the final time.