He could technically be signed now, he jut cannot be registered until the window opens on 1st Jan. This is possible as "future signings" can be made, all paperwork completed etc, the top teams do so all of the time.
This would mean that if he is signed as soon as the window opens, then 1 full window will have passed by the time the January window closes, so he could then be converted in the summer of 2027 when the registration window reopens.
Obviously this relies on Rovers being organised and efficient, so is pie in the sky stuff