Then you could use the Active Directory to grant that machine access(in a domain env) to the shared network drive where you are trying to do the write file. I think your windows server team should be able to help you with this. In the AD you would see something like $webMethodsHostname or webMethodshostname$ assuming webMethodshostname is the server that has the IS Installation.
If i were you, i would create a service account in the Corporate Active Directory for webMethods servers and then use that service account in the logon tab for the IS Services. If you have to move your code up the environments, you will have to grant all the boxes involved access to that shared drive. If it was a service account you could just grant access once and get it over with. For this scenario to work, the service account will have to be added to the webMethods box users and groups section.
I think it would be better if you have two service accounts, One for Prod env and other for all non-prod env. Assuming that the write path would differ in two environments.
cheers!
#Flow-and-Java-services#Integration-Server-and-ESB#webMethods