The solution for you is to use internal number assignment and the system will do everything for you. Now, for some reason, you want to use external number assignment. As Kiran said, you will have to write code to do so. One possible place you can enhance is the FPM feeder class for the BP number UIBB. You can enhance the GET_DATA method there by assigning a value to the field and making it read-only so the user does not change it.
You can try it and see if it works for you. Again, the recommendation is to use internal number assignment properly. You can use separate number ranges for different BP groups.
I would encourage you to revisit the requirement and understand if it is really necessary to go through all the trouble before making the FPM feeder class changes/enhancements.