These is no rpm2tar in Slackware. However, rpm2tgz, rpm2txz and rpm2targz are all part of the rpm2tgz package (not the rpm package) and this package is part of the 'a' series.
: And yes, rpm2tgz uses (and so depends on) rpm2cpio, with rpm2cpio being provided by the rpm package. the rpm package is provided by the 'ap' series.
P.S. You could have used "slackpkg file-search rpm2tgz" to find out which package provides it. Or used MANIFEST.bz2
. Just open it in less and search for the things you need.