Alejandro,
In LabVIEW 7.0 and later, you should go to the Tools Menu and choose Advanced>>Edit Error Codes. This utility allows you to create an error file (filename should be *-errors.txt) that you define errors and descriptions in. The resultant file is XML based and should be placed in the LabVIEW\user.lib\errors folder.
The ranges 5000 - 9999 and -8000 to -8999 are reserved for users.
Regards,
Aaron
LabVIEW Champion, CLA, CPI