You may not like my answer, but here goes. [Reminder for anyone reading: I don’t work for Ghost, and this is my opinion on good email practice, not telling you any specific Ghost host’s policies.]
Did these people register to receive a newsletter? Did you confirm those addresses? (i.e. by having them click a link in email) Did you used to send emails to them?
If the answer is no and they didn’t sign up to receive regular emails from you, then you shouldn’t start emailing them your newsletter. If you haven’t been emailing them regularly, then (1) you probably have lots of addresses that will bounce and (2) a bunch of them have probably forgotten they ever signed up, and some of those will report you for spamming. Both of those are really bad for future deliverability, and can get you in trouble with your hosting provider and/or your email provider.
If they signed up to get website access or to leave comments, but weren’t signing up for a newsletter, then you might instead send the following, exactly once: "Hey! Super excited to announce that I’ve moved my website to a new platform, and have transferred user accounts over. Here’s how you get access to the site now: (Add magic link info.) Now that I’ve moved, you can get an email whenever I post, so that you never miss a post! (Add detail about how often you’re going to post.) If you’d like to receive these emails, here’s how: (Include link and instructions.)
And that’s it. If they subscribe, great. If they don’t, then you don’t send them your newsletter. In other words, I’d import them as UNSUBSCRIBED members, and let them opt in. Doing much more than that is starting to veer into the land of spamming. Don’t go there.
None of that advice applies to folks who’ve been regularly sending a newsletter and are just switching platforms, as long as your newsletter list had a functional unsubscribe mechanism and was double opt-in. Those folks can be imported to your new site as newsletter subscribers, as long as they didn’t unsubscribe on the old site. (It’s going to be important to get this right.) You might want to send them a “hey, things look a little different now” message, but they’re expecting to get email from you, so it’s all good.