Can i make a public app also a CustomApp at the same time?

I developed a public app for our customer to use. Already published on App Store. Is there any way i can make it a "CustomApp" at the same time? The purpose is i wisht to push it through to all my employees device without creating tons of Apple IDs. Also, is this "custom app" only available for DEP devices?

Can i make a public app also a CustomApp at the same time?
 
 
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