Can I upload iOS app with different account's certificate/provisioning profile?

Question

An app is developed for a client and we are looking on how to sign and upload it. I have enrolled to the developer program and told client to do the same, so both are individual accounts. I see iTunes Connect allows to add another user:

That allows client to add developer to the iTunes Connect. So can I upload an app signed with my provisioning profile, all the certificates to his iTunes Connect account?

Answer 1

Answer is no. In order to upload the app and make it under your client account, then your client should upload it. You can upload it under your account and later do app transfer to make it under your client account, but if you ask me, if your client has a developer account already, send him the project, explain him how to set its cert/prov in Xcode and upload the app (or ask him permission to do it for him). That way everything will be under his account to begin with.

Answer 2

First of all, only company accounts can add users. Individual accounts — as the name suggests — can't. Any individual account can be converted to a company account free of charge provided that you have a registered business name. It takes about a week. See this page.

If you have a company account, you can indeed add users and allow them to sign app on your behalf. These users will not use their personal signing assets, but those of the company account that grants them access.
Access is not given on iTunes Connect but on the developper website (developer.apple.com).

Giving access on iTunes Connect allows to upload already signed builds and distribute them. So if you want someone to do all the work, you need to grant them access on both sides.