If your spouse agrees to file a joint return with you, you can amend their separate return in TurboTax to change the filing status and add your information and income, or vice versa.
If you used a different software to prepare the original returns, you would have to recreate both of them exactly in TurboTax, and then you can amend using TurboTax.
Once you file a joint return, you can't convert back to separate after the filing deadline.
See this TurboTax Help Article for more information and instructions.
Before you can file an amended return, the original returns will need to have been processed by the IRS (refund issued / payment processed). See here for more information on preparing an amended return with TurboTax.