Can I import a return from a different username?

[Critter]

Yes  but not directly ... it is a 2 step process.  

1) save the .taxfile from that other account to your computer 

2) transfer (not import)  the file to the online current account

2017 Downloaded to 2018 Online

Last year's tax data file (.tax files) can be transferred to Online Deluxe and higher-tier versions.

1. Sign in to TurboTax Online, if you haven't already done so.
2. Select Tools on the menu bar lower left on the screen.
3. In the pop-up window, select Transfer last year's TurboTax return from your computer.
4. Read the warning and then click Continue (or Cancel to abort the transfer).
5. Browse to where you saved your .tax2017  file and select it.
6. ClickTransfer Return and wait for your file to finish uploading.
7. When you see Transfer Complete, click Continue to start working on your return.

https://ttlc.intuit.com/questions/1900883-transfer-last-year-s-return-into-turbotax-online