To create a profile property that uses a drop-down list set the customProviderData section as below...
<add name="DropDown" customProviderData="DisplayName=Drop Down;IsRequired=true;Order=1;
DisplayType=DropDown;Options=One,Two,Three" />
Specifically, the property type must be a string, contain a DisplayType=DropDown attribute and provide a list of drop-down options with the Options attribute.