You can't add a desktop return to the Tax Timeline.
It is possible to add a file to the section called "MY DOCS", which is an online storage area. I'm not sure I would do that, however. You may have to keep using a
paid edition of Online TurboTax each year to maintain access to MY DOCS. So as far as I know, if you don't continue paying each year, you could lose access to any documents you add.
https://ttlc.intuit.com/questions/1898861-what-is-my-docs
https://ttlc.intuit.com/questions/2263274-how-do-i-upload-documents-to-my-docs
If you archive the file yourself on your computer and/or any external storage media, then you have complete control over it. I usually archive the tax data file *.taxYYYY, and 2 PDFs--one PDF of the return "as filed", and one with "all forms and worksheets."
NOTE: Remember all PDF tax documents and data
files are very sensitive files, since they contain your personal ID
info, financial data, and possibly bank account numbers, etc. Be sure
to store them safely and securely to guard against computer theft,
hacking, etc.
