Yes you can.
A buyout is an agreement supplemental to being waived. Player option years almost always have a clause that they guarantee upon being waived. So you could (but they won't) make an agreement on reduction of payment for each year, then waive him, the year guarantees, then the agreement kicks in reducing the $ to the agreed amount.
When can Kemba opt in? If it is at the end of the 21/22 season, I can't see how he can opt in and then demand a trade/buyout. At least not until buyout season. At that point, who cares.
If Kemba is on this team next year, I think he's here for 22/23 too. I'm guessing the C's will try to move him this off season and if they can't, he'll shut up and play good soldier until his contract expires.
I'd guess there's also a world where he does ok this coming season and gets like 3/60 and decides that's better than staying with the C's. I was hopeful of that 4-5 months ago, not so much anymore.
If there's a 3rd star to be had this offseason, I'd gladly staple the 16 and another future 1st to Kemba to get a 2 year head start. I guess that avenue would also be open after the 21/22 season too though, and the price to move Kemba would be less. If not, I'm fine with riding his contract out and having tons of salary room to build around the Jays after the 22/23 season.
This team really can't become a true contender until Kemba is gone.