FIRST CAD Library has a gearbox, although it might be the older one.
I wouldn't recommend running the joiner into a single planetary unless you have some calculations backing it up. The gearbox doesn't like torque that much based on experience (they have made improvements, but I still think overloading is not recommended).
Can't say the joiner is easy to find, I may have a copy of the CAD somewhere, but its not easily accesible, and its for the old version. If you don't have any luck elsewhere, PM me and I will take a look for it.
PS: Seriously consider going for the AM toughbox instead. Most applications are served much better with it.