See this TurboTax support FAQ for entering income and expenses for self-employment - https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/enter-schedule-c/L5Fz3j5us_US_en_US?uid=l9dctkeb
My wife & I filed jointly this year in April.
When you say you "filed jointly" do you only mean you began a joint return, but did not actually e-file it? Or do you literally mean that you have already e-filed a joint return several months ago?
If you did not file the joint return, then of course you can go back into the return and add the self-employment income. You can click Add a State to get back in to the return to add whatever information you still need to include or change. You do not really have to add a state return, but that is a workaround that will let you back in.
But....if you actually really filed the return in April, then you have to wait for the IRS to fully process that tax return and then amend to add your spouse's self-employment income.