For pidgin, you'll first have to install meanwhile (a slackbuild for it can be found here.
After installing meanwhile, go here
and download all of those files into one directory. Open pidgin.SlackBuild in your preferred text editor, and scroll down til you see a line that says
Change the disable to enable, save it, and compile the slackbuild.
Then just upgradepkg --reinstall on the package you just compiled, and you'll have pidgin with sametime capability.
The same basic procedure should work for kopete, but I don't use kopete so I can't say for sure.