I'd like to insert a line of text into an existing Word document that is stored in a document library. Would this be possible to do through an event or workflow? If so, how would I go about accomplishing this?
I would first write the code to modify the Word document. Assuming that you're using Office 2010, you can use the Open XML SDK, you can find specific instructions here.
Now you have to find a place to trigger this code, and you could do an EventReceiver or develop a workflow action. If you only need to do this whenever the document is added to the library the EventReciever will be the easier solution.