If I understand your question this is what you have to do.
installpkg installs precompiled binaries into the directorys given to it during "configure". In an "offical" Slackpackage this is determined in the SlackBuild script. Usually the executibles go to /usr/bin, the library files to /usr/lib, documentation to /usr/doc, etc.
To change this, for example you want everything to go to the /usr/local tree, it needs to be specified by editing the .SlackBuild script then recompiling the source with that script.
If you want to make the package your self use a prefix flag when running configure. Such as ./configure --prefix=/usr/local, then run make, then checkinstall.