This can be done with a ResourceProxy
where:
ResourceType
=SearchService
mimetype
=application/sru+xml
e.g.:
<ResourceProxy id="d55">
<ResourceType mimetype="application/sru+xml">SearchService</ResourceType>
<ResourceRef>http://cqlservlet.mpi.nl/</ResourceRef>
</ResourceProxy>
For a complete example file see: http://www.clarin.eu/cmd/example/example-cgn-sru.cmdi