PARs are scoped which means that packages exported by bundles in the PAR are only available for bundles in the same PAR to import. Similarly, services are scoped unless you explicitly add an attribute, described on another thread, to make them "punch through" the scope.
So the nearest you can get to the scenario you describe is to make sure the package of the service interface is exported by a bundle outside the PAR so that the PAR bundles and bundles outside the PAR can import that package.
Glyn Normington
SpringSource