Would it be possible, at a later point, to upgrade/convert an existing sticker pack into a fully fledged iMessages app?
According to Apple, the answer to that is yes:
Can I update my standalone sticker pack into a standalone iMessage app?
Yes, you may submit a sticker pack to the App Store for iMessage, and later submit an iMessage app as an update to the existing app. Before doing this, be sure to understand the App Review Guidelines. You can find information about Replacing Your App with a New Version in the iTunes Connect Developer Guide.