Hello - this is not strictly a direct answer to your question (but I will come to that) ..
I primarily use scribus for exactly your scenario - transcribing and presenting (old) music manuscripts (1260 pieces in my current manuscript). I generate each piece of music as a separate image file then create image frames in scribus that show the (eps) images. This gives me complete control of the printed page in scribus and allows me to automate titling, layout and indexes. It does require fairly heavy use of scripter for the automation but it means that whenever I regenerate the music, my scribus doc auto updates. For
reference I don't use musescore; all of my music work is in abc - you'd have to evaluate whatever musescore can generate in terms of images for this approach. Master pages and styles are a key component in this as well.
Having said all of that - if you open your multipage PDF you then save that as a scribus doc. Every time I've done this the PDF content on each page appears as a single grouped object which you can then ungroup and edit as required (or copy into another doc). Once you have this, open (or create) your target document and import *pages* - I'm not in front of scribus at the moment but I think it's under the pages menu. The downside of this approach is that if your PDF is regenerated you have start from the beginning. For me, solving exactly your need, I also wanted/needed to use scribus for the page layout and text elements.
cheers
Rob