Adding auxillary fuel tank to DFs Archer II a/c

Hello I will need some help with Dreamfleet's Archer II aircraft. Specifically I will need to add a auxillary fuel tank, like some collapsable tanks found in ferry flights which are installed in the back seats. The most important part is keeping the CG as close to reality as possible. If anyone can help, I would GREATLY appreciate it, since in less than three weeks, I'm about to embark on a round-the-world flight with such an aircraft based on a real flight.Thanks in advanceAris KingAthens, Greece