Bell will link with Ulster after the 2025 Test season and return to Sydney and the NSW Waratahs before the Wallabies' 2026 international program.
"To play a season for Ulster in the United Rugby Championship will provide me with new experiences and skills which I look forward to bringing back to Australia with me," 24-year-old Bell said.
The loosehead remains a linchpin for the Waratahs and his departure comes less than a week after coach McKellar said the under-achieving franchise needed a cultural overhaul.
McKellar, though, believes the short-term pain of losing Bell will ultimately prove fruitful for the Waratahs, who must win their remaining two games to have any chance of featuring in this year's Super finals.
"We're supportive of Angus's decision to continue his personal growth," McKellar said on Thursday.
"He's a young man who came straight out of school into the NSW Waratahs system, and we think the time at Ulster will be good for his development on and off the field, which ultimately will help the Waratahs long term.
"We've had a focus on creating depth within our squad this year and Angus's time in Ireland will provide opportunities for others who are willing to continue to work hard to earn opportunities in the sky blue jersey."
More of an issue for McKellar is Bell's workload in Belfast, especially with Ulster's home pitch at Kingspan Stadium being a synthetic surface.
The 24-year-old has missed much of the past two seasons battling toe injuries, chiefly the dreaded "turf toe".
Firmer playing conditions could be a problem but, while McKellar has faith in Ulster's management, he said the Waratahs would liaise closely with the club during Bell's northern hemisphere stint.
"What we need to do is make sure we've got really close connections and and regular communication with the Ulster S and C (strength and conditioning) staff and medical staff," McKellar said.
"They're going to want him on the park as often as possible in terms of when they want to pick him, so they won't want to break him either.
"But it's going to be something that we'll need to do in collaboration and and work closely together and make sure that 'Belly' comes back in one piece.
"He's been resilient this year, done a lot of work here (to get back)."
Several other clubs in the URC premiership play on astroturf-like artificial surfaces but McKellar is confident Bell will be in good hands.
"Over there, you manage your squad as well. They've got big squads over there as well," said the former Leicester mentor.
"So it's not like Angus is going to be playing 70 minutes every week. You've got 45 to 50 players in your group over there, so there's a few more games.
"I can only talk from the (English) Premiership. With three of the teams going into administration, the premiership's 18 rounds now.
"The URC play a few more but they manage their plays well over there, in Ireland in particular."
