Can I e-file a 2019 amendment if I filed the original using different software?

not the easiest thing in the world.  you'll need the TT desktop software because the online prep is closed.   you must start from scratch and make sure every number on the tax forms and schedules are exactly as originally filed. then you can amend.  then you will have to mail in your return.  it could take months until you get any refund. 

I was in the US for 2019 with my family, and we applied for an ITIN for my wife, but the IRS took months to get back to us with it (we still haven't received the paperwork), but we were able to call and get it, but only after I'd had to file individually.   That was a mistake ... the 2019 return should have been sent WITH the ITIN application.  Now you cannot amend until you get it.  

Am I able to e-file an amendment to add her details (happy to pay for it), if I originally filed using different software?   You can amend the return once you get the number however at this time efiling it may not be allowed because TT closes the efile system normally and we have no idea if they can/will keep it open later.  ALL efiling of any kind is shut down by the IRS for 6 weeks from the end of November to mid January.

 It is possible to amend a tax return prepared elsewhere using TurboTax but it will be a real pain.  It would be much easier to use the software used originally to prepare the returns than to use TurboTax.  In order to use TT software, first you have to purchase the desktop software for that year and download it to a full PC or Mac.  Then you have to re-create the tax return EXACTLY as it looked when sent to the IRS.  You then tell the software you will file by mail in order to "trick" the software into thinking you filed.  THEN, you use that same software to prepare a Form 1040X in order to amend the return, and print, sign and mail the Form 1040X to the IRS.  It takes the IRS about 3-4 months to process an amended return.  It is too late to receive a refund for an amended return for any year earlier than 2017.

To file a return for a prior tax year

 If you need to prepare a return for 2017, 2018, or 2019 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return.