Return to Deck Trait Limitations

I was trying to set up a Return to Deck trait with the name of the deck carrying a variable. For example

$JohnSide$_Discard

My intent was to send the card to the player side’s discard pile for the player side designated as JohnSide. However, I discovered that you can’t edit the destination deck in the Return to Deck trait, i.e., you must select an existing deck.

Send to Location traits allow for variables and I feel the Return to Deck trait should as well.

Question: can I edit the build file so that I can establish a variable deck location? …or will this screw it up?