I have an ASP.NET application with Forms Authentication. There is a users table in the DB with Id, UserName, PasswordHash and PasswordSalt.
Is it possible to migrate these users over to a fresh Thinktecture IdentityServer installation while keeping their existing credentials?
You don't migrate users TO identity server. You would rather implement a user service to connect identity server to your existing user store.
see here: https://identityserver.github.io/Documentation/docsv2/advanced/userService.html