-- Last Updated: Jan-11-13 8:30 PM EST --
But you should probably add a thwart between the center seat and bow seat. It would be best to suspend the seat from the gunwales on hangers. A suspended seat will not add as much rigidity to the boat as a yoke mounted directly to the gunwales will.
You also need to decide where to mount the seat. If your daughter and her friend are much lighter than you, you might use the existing gunwale holes from where the yoke is mounted for the front cross piece of your center seat frame. This will put the center passenger's center of mass forward of center in the boat which might be what you want for trim. If you wanted to use the center seat to paddle the boat solo, you would want to put the front of the center seat several inches (4-5" if kneeling, less if sitting) to keep the boat in trim.
Dagger originally made the Reflection 15. They offered a center seat as an option to allow the boat to be paddled solo. This is a link to an archived Dagger catalog: http://dagger.com/assets/images/uploads/files/DG96.pdf
If you go to page 11 you will see a photo of a Reflection 15 with a center seat. Note that the seat is canted (front of seat lower than back) to facilitate using it for kneeling. You can also see the thwart between the center seat and the bow seat.
If you want to use the center seat to paddle the boat sitting you probably want the seat level and a little farther forward. If it is just going to be used to carry a light passenger, you might mount it as I originally suggested. Mount the bow thwart far enough forward to allow leg room for the center passenger if sitting.
You can get reasonably priced canoe seats and thwarts from Essex Industries and Ed's Canoe.