If you've previously filed a tax return through another e-file service, can you amend a federal return with TurboTax?

You can but it might be hard.  You should really try to use the company that filed it.

To amend a return you did somewhere else, you would first have to fill out Turbo Tax like you are doing the original return and pay for it.  You have to recreate the actual wrong original return.  Then when it matches each line that you really filed you have to tell Turbo Tax you will mail it so it thinks it was filed.  Then you can amend it.