Is it possible to trigger a saved search email alert when a new line is added in an existing sales order?

I'm going to say No to an alert from a Saved Search as you. You could use a workflow (or script) that will trigger After Submit to compare the item sublist line count of OldRecord to NewRecord

Thank you @Netsuite Tragic Are there any SuiteAnswers articles on this? I had the same idea but couldn't find the value 'NewRecord' from the workflow setup

The values you get will be newRecord so you won't need to specify that.