The initial meld can only use tiles from the player rack. So it is not possible to extend the turn with tiles from the board. This is clear from the Rummikub rules:
The initial meld:
In order to make an initial meld, each player must
place tiles on the table in one or more sets that total at least 30
points. These points must come from the tiles on each player’s rack;
for their initial meld, players may not use tiles already played on
the table.
The other turns:
On turns after a player has made his/her initial meld, that player
can build onto other sets on the table with tiles from his/her rack.