Was Starkiller Base a rogue planet, wandering through the galaxy/universe?
The planet could be similar to Zonama Sekot,( a sentient world born from the homeworld of the Yuuzhan Vong, and capable of hyperspace travel.)
Depends on your definition of Rogue Planet. If any planet equipped with Hyperdrive capabilities, then YES.
From Pablo Hidalgo's Twitter:
The Starkiller is mobile. Able to travel distances in hyperspace amid a very populous star cluster.