question about radars

How hard would it be to make radars which would, in each pulse they normally emit while they scan the skies in track while scan mode, also send coded information? It doesnt have to be a lot of information, just a sentence or two worth of data, but it does have to be sent within its normal signal, not by a separate emitter/signal and it does have to be sent ALL the time.

At the same time, the same radar would have to have a receiving end which would receive the normal radar return plus the coded information. It doesnt matter if it uses more than one sensor for that, though i guess itd be easier if the same sensor gets all the radars signals and process them.

Also, would the signal somehow change if being bounced off something? Changed in a way that would screw up the information and the message within the signal?

If that is not hard to do, why dont we have such radars in use today?