If you are talking about a script that creates a custom bbcode that can be imported into any software that is probably never going to happen in my lifetime.
If your talking about a script to convert recognized strings into bbcodes with replacement text you are going to be hard put to find something "out there".
What exactly are you trying to do? Make a custom bbcode?..if so to do what. If you give a little more information I may be able to help you. Then again I may not...but I can't try if you don't help me help you