No you cannot add another schedule to a filed tax return. The tax return would have to be amended if you need to make changes.
A completed, filed and accepted tax return should only be amended after the tax refund has been issued or the taxes owed have been paid.
Schedule SE is created and completed by the TurboTax software if you were self-employed, entered your self-employment income and expenses using Schedule C and had a net profit from the self-employment income.
Look at your tax return. Is a Schedule C included and is there an amount entered on Schedule 1 Line 12?
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)
You would have to amend. Why didn't you get a Schedule SE? Did you fill out a schedule C for self employment income? You need a Net Profit on Schedule C to generate a sch SE. There are a few other things that go to Schedule SE. You don't normally add sch SE yourself. It is automatically filled out if needed.
Did you get a 1099Misc and enter it as other income but need to change it to schedule C income?
How to amend an Online return https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577