How do I migrate existing AWS IAM users to AWS SSO cross-account?

Currently, I have a bunch of IAM users in another account (not tied to AWS SSO).

I've recently started using AWS-SSO to manage multiple accounts and users. I found it very effective and easy to manage.

Question: How can I move/migrate users from another AWS account to AWS SSO in my current account?

I've found a few resources on how you can move an account to an organisation unit (OU) but I do not see any local users in the AWS SSO account.

You cannot do this. You will have to either add the users into AWS SSO or integrate with an identity provider (Active Diretory, GSuite, etc)