I've started doing that. I'm not adept at creating SlackBuilds, so I'm not sure what all I need to change. I've looked at the templates and read the wiki, but in this case, I'll be modifying a copy of one that someone else built.
The final package name has to be different, or I won't be able to install both packages. So I need to change the name of the SlackBuild script, the names of desktop and png elements that were supplied with it, some of the directories on the configure statement. I think I will need to repackage the tarball so it has a different name and extracts to a differently named directory during the build, since a number of things in the SlackBuild depend on the $PRGNAM variable. There are some directories listed on the configure statement in the script that seem obvious to change, like prefix=, libdir=, sysconfdir=, mandir=, docdir=, localstatedir=.
Once I get the package built, how can I inspect it to ensure that it won't overlay any of the original package once installed?
Is another SlackBuild necessary or even helpful? Would it be better to just extract the tarball, patch it, and do a ./configure and a make and make install? If I do that, how do I control where it installs to?