Out of the box CRM functionality using the Sales for Outlook Client you are only able to create Word templates using out of the box fields.  You are not able to merge into Custom fields. 

 

What you can do however, is from a List view within CRM, export the list to Excel, making sure the custom fields you want are in the list view, save that excel spreadsheet and then do your Word Mail Merge.  Let me know if you need more assistance with this.

Amy