Well, not quite (AFAIK).
There isn't likely going to be any README/INSTALL files if it's a slackpack tarball archive. You can unpack it still, and just copy the contents of the directories into the directories they are listed (such as lib) HOWEVER...
That's really not a good idea, you'd be better off using a standard tar.gz tarball to make and make install with. As for "installpkg" you have to be root to do that. And yes, you can just install it from the slackCD, but it should have been installed by default (pkgtool is sorta the de facto in slackware package management).