Welcome to Scribus! 
Importing pdfs into Scribus is not the ideal situation - but it can be done by creating an image frame (keyboard shortcut "I") and importing each page into a separate frame (Ctrl + I). AFAIK, there is really no other way to do it. The obvious problem with this solution is that the pdf is treated as an image - therefore, the text in the final document won't be searchable.
~Christen
If "Embed PDF and EPS files" checkbox is ticked in "Save as PDF" dialog, text from included PDFs would be treated as text and thus searchable, and vectors will not be rasterized, but rather remain vectors.
javierdl, if by "working layout" in Inkscape you mean creating design elements, background or draft of a page which are starting points or inputs for creating final layout in Scribus, that's fine. But keep in mind that Inkscape is still just a vector editor and hasn't much of a typographical or layout capabilities. Depending on complexity of the design you create in Inkscape, you can try saving the file as SVG and import it in Scribus via "File > Import > Get Vector File". You can even edit imported shapes in Scribus. If that doesn't work well, then do as Christen suggested.