I agree with mhearn's comment about not mixing packages. I would not recommend it generally.
However rpm is included with Slackware:
rpm-4.0.2 RPM is a tool from Red Hat Software used to install and remove packages in the .rpm format. When installing RPM packages on Slackware, you may need to use the --nodeps and --force options. Before installing any binary package, it's wise to examine it to see what it's going to do, and if it will overwrite any files. You can use rpm2tgz to convert .rpm packages to .tgz packages so you can look them over.
I am not familar with the mentioned rpm2tgz application. Perhaps it is worth a look.
My suggestion remains Slack packages or source, though.