If I’ve already filed my return for this year, can I amend my tax return to add dependents to my 1040-X? Can I also add a dependent on a prior year?

Yes. You can file an amended return this year and you may add dependents to any of the past three years returns or within two years after the date you paid the tax, whichever is later. To add or change the number of your qualifying dependents, you must do the following:

Print and mail all amended returns. You can’t e-file Form 1040X.