Using a Joker with two tiles from my rack and one from the board
My rules state:


  The tile used to replace a joker must come from a player's rack and not from the table. A joker that has been replaced must be used in the player's same turn with 2 or more tiles from his rack to make a new set.


So, playing with my set of the rules, no, the example above would not be legal. The new set would need to stand alone. Adding that '7' midstream would not be part of the 'two or more tiles from his rack to make a new set.'