You should use a sdxtool that can be downloaded as a kit from equi4 . For example, to list the contents of starkit:
tclkit sdx.kit lsk $yourfile.kit
And unzip it (into a directory with a name yourfile.vfs; you cannot control it):
tclkit sdx.kit unwrap $yourfile.kit
Starpacks , starkits , Tcl ( , tclkit ) .