Yes. Once the design type is aged enough you can add the letters.
You don't use them on radio call or in searches.
X- experimental, R-Restricted, L- Limited, C- Classic
45-22 CFR
The U.S. registration number of the aircraft; or
(ii) The symbol appropriate to the airworthiness certificate of the aircraft ("C", standard; "R", restricted; "L", limited; or "X", experimental) followed by the U.S. registration number of the aircraf