Quick and short.

1. Download mangas from OneManga using Suksit’s script, all mangas are archived in CBR
2. Change manga’s file extension to CBZ
3. Use calibre as an ebook organiser instead of Reader Library
4. Import those CBZ files in to calibre, edit its metadata as you wish
5. Send to your Sony Reader (mine is PRS-300). calibre converts your CBZ file to ePub on the fly
6. Voilà!