Hello,
is there a way to create an email account and register it at os level so it can be used for authentification to an app? I would like to buy the Yatse app with paypal but it needs an registered email.
Info from the yatse site:
The Pro version will only work on Android devices where this email is registered as an Android account.
You don’t need any account, but a gmail account. The website says:
The license is tied to your Google account
So you will have to register a (dummy) account at gmail. 
Nothing wrong with a dummy accounts. I have several of them. I normally set them up using a surplus phone that I plan to put a CR on. So long as you only give false information, share no data, and only use it for stealing, it’s a cost as far as they are concerned. They get nothing they can sell. For example my windows name and address is… Robin B’stard, msizapos@outlook.com someone beat me to ‘msisapos’. That one was set up on an old XP laptop, but you get ,the idea. On scroogle I’m westerninterloper which is kind of appropriate given the circumstances.