Star Wars is full of weird and wonderful stuff. I'd say if you really wanted some kind of transforming droid/backpack hybrid, make it like a mini arc mission. Steal some blueprints from a mad scientist, collect the parts and then something like a daunting mechanics check. Sorted.
In terms of space, weight etc, that can be sorted narratively (if your group cares) by gathering something like a new lightweight allow, or a state of the art propulsion system, stuff like that.
In the clone wars there is a droid with a built in jetpack with no real increase in size, so I'd say it's definitely doable:
As a roleplay game, you can do whatever you guys want to really, think of the films etc as source material, use what your table wants. Rule of cool is cool for a reason. I've got one player looking to make a droid exo-suit people can wear. May not be feasible, but if he gets the parts I'll let him try.