There is a way to add more keys to your 5320 phone, by using an Alternate Keymap and switching between keymaps. This is how I do it:
In programing, change the label on Feature Code 399 from Switch Keymap to Next Page. Assign that feature to one of the three non-lighted keys on the 5320. While you’re at it change Feature Code 4 from Reverse Transfer to Pick Up.
Program a new keymap, with the above features assigned to non-lighted keys. You have eight lighted keys which can be assigned as DSS keys for specific extensions. Assign this as the Alternate Keymap on the desired phones.
To answer a specific ringing phone, press the key labeled Next Page, then Pick Up, then the desired DSS key that is flashing. It’s not as awkward as it sounds, and certainly no more difficult than switching between pages on a 5330.