Yes you can
Adding your daughter as an authorized user to your existing credit cards will build her credit.
The accounts will show on her credit report and will cont in her credit score.
This kind of practice is called Credit Piggybacking. It is fairly common with parents who want to give their children a credit jump-start, and it can certainly be done under 18.
As to getting better rates on her college loans – I wouldn’t get my hopes to high. Score is only small part of the lending decision and what’s on her credit report is much more important. Having only authorized user accounts may not be enough to satisfy lenders.