it can be done, see Forwarding/Route and filter data and CLONE_SOURCETYPE in Transforms.conf . But be warned, it becomes complicated and cumbersome if your rule set is large. You may look into Splunk Data Stream Processor, DSP or also a certain third party product, for a solution which scales.
props.conf:
[thesourcetype]
TRANSFORMS-route = route_to_third_party
transforms.conf:
[route_to_third_party]
SOURCE_KEY = _MetaData:Index
REGEX = ^(winevent)$
DEST_KEY=_TCP_ROUTING
FORMAT = mysplunkinstance, thirdpartyinoutputsconf
Perhaps?