Yeah, so, AD isn't like SQL Server in that way. When you add a property to a class, you make it available for use – but the directory doesn't automatically "attach" the property, with an empty value, to all existing objects. So until the value is there, the property doesn't exist. Some tools will fake it out to make it look more consistent, but the AD cmdlets don't.
You could try using the older [ADSI] interface, or the Quest AD cmdlets, instead of the MS AD cmdlets.