I have just started to get back into chess from a long time away. I have joined my local club and started to compete. I have been playing around with SCID vs PC and wonder if you can link your opponent's online names under their real names, since I have been playing some people online and OTB. I am also considering investing in Chessbase, so if this is a feature available on there or any other chess database please let me know.
The feature you are describing is not built into either SCID or Chessbase natively. You can quickly rename players in a database, or alternatively automate through scripting.
I use the following API endpoints for chess.com to collect that information. https://www.chess.com/news/view/published-data-api#pubapi-endpoint-player