I'm not a cap expert, but my understanding is that we could only get him via trade. There is no way we could sign him.
Lowry is a free agent and so is Fournier.
So, as far as I understand, to make this work, Lowry and the C's would have to agree to terms and want to do this, and Fournier and Toronto would have to agree to terms and want to do this.
Toronto would also have to want Tristan Thompson, and possibly some additional filler. And have no other teams lure either Lowry or Fournier with a better offer. Then I think Lowry could sign a deal with Toronto, Fournier could sign with Boston, then we could make the deal.
So, it is possible but extremely unlikely.
What will Lowry attract for a contract? He's 35. Both money and years are a big deal.