Now I have found a way to use SU Layout also for other documents having pdf's:
I have a pdf with paper size and scale as needed. Then save this image as a png file (with a high dpi resolution like 300). Then make the white color (for example the paperbackground) transparent. Then import this png in SU Layout, paper size must be same as the original pdf. Then scale the png in the upper left and under right corner. That'it, now the size is like importing a pdf.
Maybe ohters can use this too...
Screenshot2012_08_16_14_09_57.jpg