You can do this already, you just need to publish the field.
Go to Custom Fields. Then scroll down to "Comment" and click on it to edit.
Change the following so it shows just in the back end to you:
- Published: Yes
- Show on subscription form: No
- Show on profile: No
- Show on Sub... Manage...: YES
- Hide on renewal: Yes
- Hide on Email: Yes
IF you already use the Comment field for something where the user can leave a comment, you can always Save a Copy, and then rename the copied field title to something more useful, maybe in this case "Extension", and then use the settings above.