Kpackage uses the command related to the package you intend to install.
For a rpm-package this will be 'rpm -Uvh'
And rpm looks for dependencies/installed rpm's in /var/lib/rpm only.
Meaning that everything will be missing.
If this package is something, you cannot get as a slackware package
or cannot compile yourself, it might be installed with kpackage.
You just remove the x in the button saying 'check dependencies'
(You should install genuine dependencies before packageinstall)
To keep things simple in your system, there is a better way:
change the rpm-package to a slackware-package with 'rpm2tgz'
then install with command 'installpkg' or use pkgtools.
This way the packagename/filelist will be added to /var/log/packages,
the slackware 'install-log'