You will want to create a List Definition and then set that as the source for a Manual property. If the Help topics on this are insufficient, you may find this blog article of interest:
http://architects-desktop.blogspot.com/2006/03/enhanced-property-data-features-in-adt_31.html
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David Koch
Autodesk Discussion Group Facilitator
Using ADT 2004 at work; access to 2005, 2006, 2007 & 2008 at home