I dont think you could do that. It takes the native editor that is embedded with it.
You can changed the editor you use, however I believe it’s read only. I thought there was a section in the user guide on using a different editor, or you should be able to look in the preferences.
For a blpackage external command, you cannot use an external editor.
You can import a file from a server , so you can edit your script/command on the system editor and then import the same.
Or even a copy paste will work.
What kind of problems are you facing with the editor ?