Sure thing, let me know how it goes.
Login to your blackberry enterprise server as the blackberry admin. (not the local admin)
Open up BlackBerry Manager
In the explorer click on "User Groups"
Then on the tabs on the top click "Global"
Then on the Tasks click "Service Control & Customization"
Then in "Edit PIM Sync Global Field Mapping"
You can now assign custom fields as you would like.
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To make sure that they are all syncing you can:
Click on "Edit Properties" (same section you are in "Global")
And then click "Global PIM Sync" and make sure they are all set to "Bidirectional"
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Note if you have problems with them entering addresses on the BlackBerries and them not showing up you might want to set the "Address Book" Conflict resolution field to "Device Wins"
Hope that helps.
Also I found this to be helpful as well:
http://www.blackberry.com/btsc/search.do?cmd=displayKC&docType=kc&externalId=KB04444&sliceId=&dialogID=199908943&stateId=0%200%2095769302