when ezfind can search on external content as defined in feature here:
http://doc.ez.no/Extensions/eZ-Publish-extensions/eZ-Find/eZ-Find-2.7/Use/Advanced-Search/Advanced-search/External-Content-Searching

the filters are used in conjunction with the regular filters used in the fetch operator (the join condition is AND). what is required is a possibility to use the filter in disjunction (the join condition is a OR) so that the results can be either internal eZ objects OR external objects.