Hey,
can you explain your use-case a bit more maybe? What exactly are you trying to solve? Is this field in the same document that you are trying to index or in another document. Maybe there is a different way to solve this. In general this is something that you would have to implement yourself.
Have you seen the ingest node functionality? Using a script processor your could possibly do this already, but again this depends on the concrete use-case, so more information is needed.
--Alex