Yes you can accomplish this.
I tested this on R10.8 by going to Voice > IP Phone Globals and then adding custom directory entries. When done, click the Save GLobal DIrectory button.
Then go to IP Phone Configs, select all the MAC configs, then go to Other Actions and select update directories.
What version AOS you running on your NV7100? There was some issues with Polycom phones in earlier releases that have been fixed.
Once way to check this is to view the MAC-directory.xml filein the CFLASH/Polycom folder after you have made the changes above to verify that the new entries have been saved to the file.
Let me know if you have any questions.