The phone user guide gives instructions on how a user might assign their own speed dials, if it is allowed - it does not say when that might be, and we did not try it, but here is the part in the guide that discusses it:

Program Personal Speed-Dial Buttons

Your system administrator can assign speed dial numbers to your phone. You may not be able to program every speed dial number.

To program personal speed-dial buttons, use the following method:

1. Select an available phone line by lifting the handset, by pressing the NewCall soft key, or by pressing a line button. Listen for the dial tone.

2. Press the pound key (#).

3. Press the speed-dial button that you want to program. A short beep confirms that you are starting to program this button.

4. Enter the speed-dial number. The digits appear on the phone display. Use the backspace (<<) soft key to erase digits that were entered incorrectly.

To remove a speed-dial number without replacing it with a new one, press the pound key (#).

5. Press the same speed-dial button a second time to indicate that you have finished entering the speed-dial digits and to store the new speed-dial number.

6. To continue, press a new speed-dial button and repeat the process. Hang up the handset to finish.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/ip_phones/7931/4.0/english/user/guide/ugKSsccp.html#wp38541