it's not easy to track changes.
and scribus cannot do it.
it's even harder if you have modified the text in both the source and in the frame.
also, if you're formatting your text in scribus, it's not easy to keep the styles when applying the changes from an external source.
depending on the way you create and edit your text files and the the type of formatting you're applying to your text, it might be possible to create ad hoc python scripts that keep the scribus file in sync with external sources. but i don't think it's trivial (or even possible) to create a general solution that works well for all (or even most) cases.
personally, i've done something in this direction for a comic book (it was about exporting all text and re-importing it translated), and if you very specific needs i can see what i can do for you.