Or better yet, avoid rpm's all together for Slack
RPM's are very cool, and work great; but for systems that are setup to use them. RPM based distros like Mandrake and Redhat are designed for this, and have an RPM database prebuilt and ready to go out of the box.
If you find you *have* to use an rpm, I'd take pesho's suggestions and rpm2tgz it first, and then go from there.
Cool