A sign-and-trade is a transaction in which a team re-signs its own free agent and immediately trades the player to another team, usually so the new team can land a player it could not afford with cap space.
HoopSim has no sign-and-trades. A player you re-sign stays on your roster like any other player, and a trade is a separate deal that has to pass salary matching on its own. What HoopSim does keep is the idea behind the rule: Bird rights travel with a player in a trade, so a team that acquires a player at the deadline can still re-sign them over the cap in the summer. If you want a free agent you cannot afford, the routes are cap space, the mid-level or low-level exception, or a minimum deal.
Rulebook: Aprons. Related: Bird rights, salary matching, trade exception.