Hi @R3dKap ,
Do you want to import a SharePoint Custom form app into your PowerApps Environment?
If you want to import a SharePoint Custom form app into your PowerApps Environment using "Import Package (preview)" functionality, I afraid that there is no way to achieve your needs in PowerApps currently.
Currently, within PowerApps, the "Import Package (preview)" functionality could only be used to import srtandalone canvas app zip package file, it could not be used to import SharePoint Custom form app (created via PowerApps) package file.
I have made a test on my side (import canvas app package file), "Import Package (preview)" functionality works well. The screenshot as below:

If the issue also still exists, please consider clear cache of your browser, sign in your PowerApps again, try the "Import Package (preview)" funationality again, check if the issue is solved.
Best regards,