Smith is understood to have completed a medical with the Rabbitohs over the weekend, paving the New Zealand Test hooker to join Souths until the end of the season.
The premiership-winning No.9 remains six to seven weeks away from a full return to fitness following an anterior cruciate ligament rupture last season.
Smith had already signed with Souths for 2026 and 2027 after a turbulent spell with the Roosters.
Roosters coach Robinson said the mid-season deal had been done amicably with the club's salary cap in mind.
But Robinson said Young's mooted exit was markedly different.
Young was dropped to NSW Cup two weeks ago and was linked to returning to Newcastle just 18 months into a four-year deal.
Robinson said: "It came out as if we were pushing him out, which was not the case at all.
"We'll get back to work with Dom... Dom hasn't done anything wrong."
Robinson claimed the Roosters had no intention of letting Young leave and the three-time premiership-winning coach then took aim at Newcastle.
He said the Knights and the Englishman's management had turned Young's "life upside down" by attempting to force a return to Newcastle and then asking for the Roosters to contribute financially to make a deal happen.
"We've not had one discussion about letting Dom go at all," Robinson said.
"Then we get approached by Newcastle and his management, and they say, 'we want this to happen'... we say, 'Fine, you take up the contract'.
"They want us to release him, but for us to pay money for it when we weren't even thinking about releasing him. In what world does that happen?
"You asked, you said that you had room in your cap, get it sorted... You threw a guy's life upside down for a week."
