Open Bidding for Google Ad Manager

Actually, it will happen like that already. When you create your Google Ad Manager child account, you will be asked to link (or create) an AdSense account (which handles the payouts).
And as you already have an AdSense account because of Admob, your balance will actually be Admob + Ad Manager.

I just checked and InMobi Open Bidding is reporting a very low fill rate. I just asked them what is going on.

If you already have an AdSense account, Google will ask you to link that AdSense account.