Hey chzburger10,
We think you are asking if you can still access accounts on a domain that you transferred to iCloud+ with the previous provider. For example, if you owned the domain chzburger10.com and your email was chz@chzburger10.com, you want to know if you would still be able to access that email through your previous provider after importing it to iCloud.
If we're correct, the answer is probably not. When you port your domain to iCloud you have to change your DNS records and update the settings with your domain register as outlined here: Set up an existing domain with iCloud Mail - Apple Support
This lets the domain registrar know to point your traffic toward your iCloud account, so you wouldn't be able to check it with any other providers.
Kindest Regards.