If you used the Turbotax downloaded versions in the past those tax files CANNOT be uploaded to the online system. You can only transfer the 2018 info into the 2019 program.
If you used the Turbotax online versions in the past, the only way to keep them together is to use the same account every year ... if you used different accounts in different tax years the accounts CANNOT be combined or merged. Going forward use only one account and the others will go dormant.
If you used any other tax program in the past then the Turbotax program CANNOT see any past returns nor can it marry them in the same account.