You want to add comments or remarks inside the scheduled scan file with a list of computers that will be scanned.
When a scheduled scan is created, it writes the parameters into the XML file toolcfg_schedulescan.xml located at C:\ProgramData\GFI\LanGuard 12\. Later on, this file is copied to cache, and during a scan, LanGuard will work with the cached file at C:\ProgramData\GFI\LanGuard 12\Cache.
What LanGuard expects to find on this file is an IP range, which it will try to resolve through network discovery. Any comments added will be reported as errors in the scan with the error message "Unable to resolve host."
In summary, the scan would still complete for the valid IP ranges found, but you would have the error messages in the scan results described above.
To suggest a possibility to add such comments in future LanGuard versions, you can submit a feature request directly to the product management group.