The Storage Planet was one of the many planets conquered by the Irken Empire. The Storage Planet was presumably at one time home to a species, until it was conquered.
The Almighty Tallest are seen on the Storage Planet in the episode "MegaDoomer", where they find the MegaDoomer Combat Stealth Mech. which is being sent to Invader Tenn to help her with her mission on the planet Meekrob. They also find many malfunctioning SIR Units, which behave like GIR, though considerably more dangerous. They decide to send these to Zim.
However, when an angry slave, Smikka Smikka Smoodoo of the Screwhead race, wants to start a revolution, he switches the names on the two packages, resulting in Zim getting the MegaDoomer and Invader Tenn getting the malfunctioning SIR Units, which leads to the destruction of her base and ultimately the failure of her mission. Like the Conveyor Belt Planet, the storage planet seems to be entirely artificial.