Multi-selection drop-down list

I have a need to be able to select one or value from one drop-down list:
Extract from sample schema definition:
 
      <xs:element name="ComnAttr">
             <xs:complexType>
                   <xs:sequence>
                         <xs:element name="CnfgGrp" type="GrpList"
default="General"/>
                   </xs:sequence>
             </xs:complexType>
      </xs:element>
      <xs:simpleType name="GrpList">
             <xs:restriction base="xs:string">
                   <xs:enumeration value="LDAP"/>
                   <xs:enumeration value="IMAP"/>
                   <xs:enumeration value="iPlanet"/>
                   <xs:enumeration value="Mirapoint"/>
                   <xs:enumeration value="Notification"/>
                   <xs:enumeration value="General"/>
                   <xs:enumeration value="Logging"/>
                   <xs:enumeration value=""/>
             </xs:restriction>
      </xs:simpleType>
 
Desired XML CnfgGrp possible value selection:.
 
<CnfgGrp>LDAP IMAP iPlanet</CnfgGrp>
 
Is there a way to define this in the schema definition?
 
Thanks,
Rick Aridi