extending an xs:attributeGroup?

Hi,

We have a base schema that is used by multiple projects. In one project 
we need to add an attribute and make it available on all elements. I 
currently have an attributeGroup in all complexTypes/elements, like:

<xs:attributeGroup name="core.attrs">
   <xs:attribute name="id" type="xs:ID"/>
   <xs:attribute name="title" type="xs:string"/>
</xs:attributeGroup>

is there some way to extend this attributeGroup so that I can make 
another attribute available? I don't want to change the base schema, but 
want the group to appear as:

<xs:attributeGroup name="core.attrs">
   <xs:attribute name="id" type="xs:ID"/>
   <xs:attribute name="title" type="xs:string"/>
   <xs:attribute name="role" type="xs:string"/>
</xs:attributeGroup>

possible?

thanks,
-Rob