No you can't link them. But you can transfer from 2017 online to your Desktop program.
Transfer from Online into the Mac Desktop program. Expand the 3rd blue link
https://ttlc.intuit.com/questions/1901265-how-do-i-transfer-last-year-s-return-into-the-2018-turbota...One way is to start a 2018 Online return and transfer into that. Then you can download the .tax2018 file from online and just OPEN it in the Desktop program.
