If we look at the waveset.dtd file we can see the attributes appropriate for this element:
<!ELEMENT IDMAttributeConfiguration (#PCDATA)>
<!ATTLIST IDMAttributeConfiguration
name CDATA #REQUIRED
displayName CDATA #IMPLIED
syntax (BAG | STRING | INT | BOOLEAN | DATE) #REQUIRED
multiValued (true | false) 'false'
description CDATA #IMPLIED
so, in the IDMAttributeConfiguration element we can specify multiValued='true'
(This applies to IdM 8 )
-Bill