Hi,
At first I would like to mention that upcoming Kentico CMS version 8 will have more customization possibilities in terms of update and rebuild process.
As for version 7, you could add an additional field to your document type in Site Manager -> Development -> Document types -> Fields. After that you will have to go to Search fields tab and make this newly added field searchable and tokenized (in case you want to apply analyzer).
Next step would be creating an event handler (
http://devnet.kentico.com/docs/devguide/index.html?event_handlers_overview.htm), this handler will be fired before a document is created or updated, in this handler you will populate the field with your data.
I hope this information helps.
Best Regards,
Roman Konicek