The only way I can think of would be to add the calendar to the Gmail account that is tied to the Droid.
1st, log in to the calendar that you want to add.
Go to "Setting"
Click the link "Share this Calendar"
Enter the address of the account tied to the Droid
Adjust permissions as desired and click "Save"
Then, log out and log in to the calender with the account tied to the Droid
In the box called "Other Calendars" enter the email address of the calendar you want
Once the calendar is added, resync the calendars on the Droid.
Hope this helps.