Do we have a package for PIL? I don't see that as a separate package in the repos whih is probably why it's not listed. It may be included in another package that does show up in your SlackBuild.
In any case, for general reference:
Basically you can add to the slack-required file the same way you build the slack-desc file, except you use the >> operand to append to an existing file:
cat > $RELEASEDIR/slack-required << EOF
<package name>= <version>-<arch>-<build>
EOF
So... if I wanted to add puthon itself as a dependency the code would read:
cat > $RELEASEDIR/slack-required << EOF
python >= 2.5.2-i486-1
EOF
Note that our python package is borrowed from Slackware. It is more common that a build will be something like 1vl60. Also note that you can have as many lines for dependencies as you like after the first reference to EOF.