Your old trim might be finished with a shellac or not, but what it does have is a patina. This will be somewhat difficult to reproduce, not impossible.
What you need to do is take a 4ft piece of the new trim and a 2ft (or more) piece of the old trim to a paint store. Personally, I would check out your local independent paint stores not the corporate ones. Let them know you need a custom mixed stain to match the existing.
OK, let say they are able to get close but not quite there. Now you need to ask if they can tint a clear coat, can be poly or acrylic urethane. This tinted finish will act as a toner to push the color further than a stain alone can do.
The toner is just that. Don't use it as the first seal coat (sand/tack coat). Use either a sanding sealer or untinted clear as the first coat, depends on the finishing system you choose. The toner will still need an untinted clear over top to protect it from scratches.
I do this all the time. Locally I have a great paint store that is willing to put in the effort and time for such a custom match. If your paint store can do this, great! Remember, they are loosing money doing this match; lots of time for a little profit. Make sure to buy not only the materials you need now but future paints as well from this store. This truly good paint store deserves your business.