Below is a bash script depending on ImageMagick 6.2.x which makes simple imposition of PNG files exported from Scribus.
Version: IMpos 1.0 11.10.2006 Copyright: Mikołaj Machowski 2006 ( mikmach AT wp DOT pl )
Usage: impos -o format [options]
-c - color of background (default: white) -d - directory for outut (default: imposout) -h - this help -m - multiply first image to fill empty sheets -o - format of transformation, arguments are 2up - 2-Up 4ss - 4-Up Saddle-Stitching 4tumble - 4-Up Work-and-Tumble 4turn - 4-Up Work-and-Turn 8tumble - 8-Up Work-and-Tumble 8tur - 8-Up Work-and-Turn
More info about imposition on: http://www.prepress.pps.com/TechReports/imposetr.html
WARNING: it takes the same order of pages as return by the ls command, but Scribus creates filenames in natural order like:
filename-page1.png, filename-page2.png .. filename-page10.png
You surely can see the danger: the listing will be in output in the "computer" order, e.g.: 1, 10, 11, 12, ..., 2, 21, etc. To avoid such problems you should attach a leading 0 to numbers (01, 02, 03, etc.). IMO it would be a good thing if Scribus did that by itself, it would help in all situations where the proper order of listings is important.
This is simple imposition targeted toward simply home jobs to fit in A4/A3 printer, without crop/fold/register marks. If someone would be interested in getting those features I will expand it. Note: bleed space will be never supported due to the nature of image export from Scribus :)