Yes you can. In the calendar, double click on a contact to open it. When the contact window opens you should see a tab called, "ALL FIELDS" Click on this tab, and you should see a drop down menu. In this drop down menu select "Miscellaneous Fileds" You should see two colums now. Column one is the name of the field and column two is the field value. The field names are in alphabetical order, go to "User Field 1" You can type in the PIN here. This automatically sync's with the BB PIN field. If you don't sync at all, insert in the notes field. And you will see it when you open the contact. You can also change the fields according to the numbers you use for the contact.
For example, some of my friends do not have a Business Fax, I change it to other, and put any other number I need for them here. If you need help with this I can walk you through it, just PIN me. Its in the sig. Hope it helps!!