Changes in master data will not automatically move silently into already existing transactional data. The will usually only flow into transactional data that is created after the master data change.
And loading additional text into existing text (extending existing text) is as well not possible.
The load programs for text (e.g. SAVE_TEXT function module) just overwrites the existing text with the new text.
So you need to read the existing text from the database into internal table and then add your additional text to this internal table, and finally you transfer the complete text to data base.