Hi @Blu3Wallaby,
First of all, please do not let the length or occurrence of strange words in this thread discourage you from trying out RoboCupJunior Soccer!
Yes, the use of OpenCV it is currently well within the regulations and so feel free to use it to put together something for ball recognition.
You can find quite a lot of materials on this very topic online – I would also like to point out that a very active member of this forum @elizabeth.mabrey has shared a very nice set of materials in this thread on OpenCV which may serve as a good starting point.
Hope this helps!
– Marek