How can we add another 1099R?

Did you already e-file?  If not.  just go to the appropriate section of the software and click on add 1099R  Go to  Federal> Wages and Income>Retirement Plans and Social Security>IRA  401 k) Pension Plan Withdrawals to enter your 1099R.

I've already e-filed but was rejected.  Do I refile the whole file or try to correct the old e-file?

Correct the rejected return.  Do NOT click on anything that says "amend"  Just go back to the section for 1099R and add the one you forgot.  Then you can re-submit the e-file.

If you have not already filed, do this - 

• Sign in to your TurboTax Online account, and click on ''Take me to my return.''
• Click Federal 
• Click Wages and Income 
• Scroll down to Retirement Plans and Social Security
• Click Update next to IRA, 401(k), Pension Plan Withdrawals (1099-R)
• On the summary screen, click Add Another 1099-R, see attached

If you have already filed and been accepted, you will need to complete an amended tax return; here are the instructions.  Amend My 2017 TurboTax Return

i tried that, and it wouldnt let me add a 1099R