I already completed 2018 tax data input, but found that I forgot to import 2017 tax, can I still import 2017 tax to my existing 2018 tax?

[Carl]

The only possible way to import from your 2017 return, is to start over your 2018 return from scratch, by deleting the .tax2018 file in the documents/turbotax directory and just "start over". You can only import upon starting your return, and there is no other option.