You can call external commands/applications. Use codes like {sourcepath} and {filepath} to pass arguments to them about the current directory or selected files.
Have a look in the tutorial section (and/or the manual) to learn more. Although it's out of date (missing some of the featured added since it was written), the External Control Codes (guide) one is a good place to start. The Nero Toolbar creation tutorial may also be worth reading as it's about running an external program. (Although the info about Nero itself is out of date now as Nero keep changing its command-line.)
My Toolbar Editing video may also be worth watching, although I don't think it goes into external commands (but much of the information still applies).