If you want to rebuild the package so that it uses xwmconfig, then the easiest way is probably to look at an example. Maybe visit the slackbuilds.org site and look at the build script for Openbox. Read the comments in the Slackbuild. (Specifically, you'll need an xinitrc file, and you'll want to add a line to the SlackBuild that will include the xinitrc file in the new package.)
If you don't want to rebuild the package and decide not to use xwmconfig, you can always just edit the .xinitrc file manually. Remove the line that starts the current WM, and replace it with something like:
