After creating a new assignment, click on the + button near the comment bank. 

This will bring up window that will allow you to import and/or link comment banks from other assignments (past and current). 

You can do three things here:

1. Add a new comment to the bank (great for "pre-populating" banks)

2. Import the comments from another assignment (without affecting the other bank. This will overwrite any comments in the current bank). 

3. Linking to banks together (so that comments added to one assignment will be automatically added to the other, and vise versa)