is there any way i can use an exisiting tag field in a web source script ?
I am not sure what you mean. You need the contents of some tag fields to define the search criteria for the web source.
no, i meant to use an existing tag field to define an output from the web source
No, I think there is no way to use an existing field value to influence the web source behaviour. Anything like that has to be undertaken as some kind of post-processing with actions and stuff.
You can access a tag field in a script, but what are you trying to do?
so basically iTunes purchased files have an xid structure (prefix, scheme, identifier) , while the Apple Music source only provides the unique identifier i want it to keep the full structure thus only updating the identifier
Then you have to adapt the script to your needs and remove all field assignments that you do not need.