that space was one of a kind, and there's no way to physically enter it now.
and while it's possible for goku to shout as hard as SSJ3 Gotenks or Super Buu and create the space rift necessary, Superman could do the same to escape.
also, we don't know if that way is possible for entry, rather than just exit of the subspace.
third, and most importantly: there is natural light there, otherwise it would be impossible even for normal humans (?) such as Tien to survive there, further more, Piccolo. He seems to some subtype of planet or whatever thing, he can consume food, but mostly needs water and sunlight. and he had no trouble staying there for a year. while pushing himself to the utmost ability.

to conclude, goku can't take them there. if he can, then superman can get out, and if he can't superman isn't weakened there,