How to add an external editor to Eclipse
Go to:
Window->Preferences->Editor->File Associations
...to register your external editor for a specific file type or file types.
Then go to:
Window->Preferences->General->Workspace and check the checkbox "Refresh using native hooks or polling"
...to make Eclipse aware of changes in the files whie you are editing. This setting is important since otherwise you will have to close the external editor in order for Eclipse to pick up the changes which will make backing out of changes with the undo buffer impossible (since you've closed the editor once you realise something is wrong).
Read more: Link - Adding an external editor in Eclipse — VIM Zone
