@ChikNg - I teach at a community college, and I have a strict policy that all work must be submitted through Canvas. I will not accept work that is emailed to me or even submitted as a "submission comment." Every single piece of student work must be properly submitted by the student, especially if you're concerned about plagiarism.
Here's my recommendation.
Reopen the Assignment for that individual student with the +ADD function. Once the student properly submits his or her work, you will instantly know whether there are any issues.
Don't mess with "work around" solutions. Don't try to run it through Turnitin yourself. It's your student's responsibility to submit his or her work.