Hi @vdubrovskyi_1854 ,
Without knowing which specific package you're referring to, it's hard to give a more specific example.
Historically, Packagist was effectively a database of "GitHub Repository pointers" and Composer was effectively just a wrapper around the Git client. They can both still operate in that mode, and needs to for older, non-standard packages. I suspect that's the type of package you're referring to here.
ProGet's Composer feeds work with "packages" (i.e. not the GitHub pointers), which account for nearly all of the modern packages on Packagist.org. For the small number of packages that don't follow the standard (mostly older, legacy packages), you'll need to download them, properly package them, and then upload them to follow the standard.
Unfortunately there's no technically feasible/sensible way to solve this problem - since it would require ProGet to serve as a GitHub pointer database, which just doesn't make sense.
Cheers,
Steve