How are you going to charge the extra battery? You either have enough power available from a 24-12 converter to run the lights or you don’t. A battery in the middle of that circuit isn’t going to really do anything for you except cause charging issues as your 24-12 converter needs to be designed or adjusted to charge batteries. You cannot really charge the battery directly from the 12v as this in effect changes the size of the batteries as seen by the alternator. 12v is the weak link on these trucks and the 4 battery config grossly overloads it if you are still running 4, drop to 2… even at 2 with the OEM lights you are using nearly all the 12v capacity anyway. I would not add any load to 12v, only to 24 using a converter…
i would simply add a 24-12 converter capable of meeting the needs of your 12v accessories and connect them all to a 12v distribution panel as you have done for your current accessory panel… victron makes some nice converters. The Orion is good for 70A(35a from the 24v side) not horribly large, remote controllable on/off via switch or relay, ~$150…
my .02