Use the environment variable approach. The other answers with putting the debug args in the -Dexec.args="" seems to just pass the arguments to the program you're executing. Perhaps that may work if it's configured to launch a new JVM for the exec.
For your sample project, I changed the Debug tool command line to "mvn exec:java -Dexec.mainClass=mavenhelloworld.MavenHelloWorld" and clicked on advanced and added the MAVEN_OPTS environment variable as "-Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000".
When you hit F5 to start the debugger, the program will start suspended, but SlickEdit will not automatically go into debug mode because it doesn't recognize this maven command as invoking the debugger. So you'll need to go to Debug -> Attach Debugger -> Attach to Java virtual machine... Port number 8000, and select your workspace as the Session name, so your workspace breakpoints will be used. Hit OK.
The target is still suspended at this point, so hit F5 again to start it up.