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]
Where options are:
-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 order of pages the same as ls is returning. But Scribus is creating filenames in natural order like:
filename-page1.png, filename-page2.png .. filename-page10.png
You obviously see where is danger: listing will be in "computer" order – like: 1, 10, 11, 12, ..., 2, 21, etc. To avoid problems you should fill numbers with 0 (01, 02, 03, etc.). IMO it would be good thing if Scribus would do that by itself, it would help in all situations where proper order of listing 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 nature of image export from Scribus :)