Er, yes you could. There were always color pools in the Astral, portals in Planescape, the plane shift spell (the 2e description of the spell says "The size and metal type [of the tuning fork] dictates to which plane of existence, including sub-planes and alternate dimensions, the spell sends the affected creatures."). The idea in Spelljammer was that there was the Prime (which contained Realmspace, Grayspace, Krynnspace, etc.). They only had their own cosmologies because the main planets had their own cosmologies, and you could plane shift from the Realms to another plane, and then shift again to Krynn. Or you could exit Realmspace, sail the phlogiston, and enter Krynnspace.
In 1e, there was the idea that the Realms, Oerth, and Krynn, as well as your setting, my setting, and Bob's setting--and Boot Hill and Gamma World, and the real world--were all alternate material planes.