Hello.
Thanks for answers, especially to Pratyush - while not perfect, it will do fine for some cases.
It is shame that plugins can't access all open files - plugin itself would be quite simple:
Calculate sum of all widths of open files (SumW)
Calculate max of all heights of open files (MaxH)
Create new image with SumW x MahH dimension
Import one by one image, shifting each one with sum of widths previously imported images
Maybe this can be added in Paint.Net itself? :-)
Regards,
Dejan