You can migrate all user passwords in encrypted format, as long as you're able to do the same check - using the same password hashing algorithm.
For example, if your "ASP DOT NET STOREFRONT" is using MD5 with salt key, then you can use mine extension for Magento2 https://github.com/sinisa86/Sinwashere_LegacyAuthentication which will allow your legacy customers to log in properly.
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