slapt-get reads the PACKAGES.TXT file for package information (which is also what it reads for a package's dependencies). Simply put, sourceforge.net does not provide this file to tell where a package's files (or it's dependencies) are.
You can create your own mirror, but that requires some work that is usually scripted. For example, in order to add a package's dependency information into the PACKAGES.TXT when you decide to put in a new package, the slapt-get documentation asks that you use a "slack-required" text file included with the package in the $BUILD_ROOT/install directory with a list of required packages that can be scripted to be added into PACKAGES.TXT.
In addition to that, you would probably want some meta files and other such tings. I think Alien Bob might have a script to assist in creating mirrors (or it was a different Slackware developer) to help with most of this stuff.