Nope...not a problem. You'll be able to add another bag. You can also not check a bag you said you would (by just bypassing check in).

Don't know the answer to the kiosk vs. agent question, but I suspect the kiosk will give you an option to change your bags (like you can in the app after initial check in). On that note, you can always go to the check in tool to amend the number of bags you are checking.

Note that I don't know how well it does with this and a payment required - due to elite status, I haven't paid checked bag fees to UA for a loooong time..possibly ever.