Short answer: you can’t. Once owned by a business manager account, a facebook pixel or facebook ad accounts ownership cannot be transferred to another account. You can share access to a pixel or ad account by adding the other business manager account as a partner, but currently ownership cannot be changed. If an ad account or pixel is owned by an individual facebook user it can be moved or shared access to a business manager account but not the other way around. So be careful where you create your pixels and ad accounts.
I suggest you create a new facebook business manager account for every business and maintain ownership of all assets. Only give access to ad managers and other marketing businesses you may work with through the partnership share feature. It’s also a goo didea to create and link a facebook pixel to your website right when you first launch your site, even if you are not running any ads, just to start collecting customer data.