Any CSS has a CSS Center. It is a special account, similar to a multi-client account in the Merchant Center. Multiple Merchant Center acccounts of different merchants can be connected to it.

Your CSS provider (or you yourself, if you become a CSS provider) needs to send a request from the CSS Center to join the CSS to a Merchant Center account, and the admin of that account needs to accept it.

Google will then migrate the account from Google Shopping Europe (the default CSS, that belongs to Google) to that CSS.

Easy as that! You will manage your campaigns and your Merchant Center account, as you did earlier. Only the CSS changes. And, of course no Google margin is deducted from your CPC any more.