I think you are on the right track keeping the exit lights on the existing emergency circuit. Anything that is part of the life safety system needs to be kept separate to what you are proposing. If you do this you can classify the new system as an optional stand by system and wire it per article 702. Separately derived and optional stand by are not part of the same discussion here. Whether it is separately derived or not has to do with grounding and bonding, not emergency or optional.
