Is it possible to upload service sims from a premade town to another?

Yes and no. We can always place copies of sims in one world into another. But in the case of Service Sims, there is no way unless one uses mods to get them into the service pool in the second world and even then no real way to guarantee that they will actually be used for their services.

What you might be able to do, although it's not exactly the same thing, is save these sims within CAS (the testingcheatsenabled kind) as Saved Sims/templates, bring the Service Sims in the second world into CAS, and replace them with these templates.

Thank you