Yes, you can buy a smaller contract at BWV, but it will be the same Use Year as your OKW contract. It will have a separate contract number, but the same leading numbers (12345.001 and 12345.002). You could sell either of the two separately. You cannot sell a portion of the OKW points, only none or all. It will not be a different member ID number as long as it has the same UY and is titled the same. If you try to title it differently, they might not let you only purchase 25 points.
You can only combine 100 OKW and 25 BWV at seven months out. BWV points will always be BWV points and OKW points will always be OKW points.