If post migration you need to add a user to be a local admin to an Entra Joined device you can log in as a local Admin yourself.  From an elevated command prompt you can run:

net localgroup admnistrators "AzureAD\[EntraIduserPrincipalName]" /add

e.g.
net localgroup administrators "AzureAD\donald.duck@contoso.t2t.dev" /add

When you look in the Local Admins Group you will now see:

You can also add this to a Post Migration script in your Runbook.