You can use this to make extend a choice group.

In types.xsd:

<xs:group name="top-level">
  <xs:choice>
     <xs:element ref="my:xslt"/>
     <xs:element ref="my:subtree"/>
     <xs:element ref="my:wrap"/>
     <xs:element ref="my:template"/>
   </xs:choice>
</xs:complexType>

The redefine:

<xs:redefine schemaLocation="types.xsd">
  <xs:group name="top-level">
    <xs:choice>
       <xs:group ref="my:top-level"/>
       <xs:element ref="my:route"/>
    </xs:choice>
</xs:redefine>