[NTG-context] crop PDF pages

Hi, powerful context users: I'm wondering why there is no crop setting for \externalfigure or \insertpages. I have a lot of scanned books in PDF format, most of them are double sided. I want to read them on kindle but the huge margins take too much space and as a result the texts are too small. I want to use commands like \insertpages to crop all the pages with different bounding box settings for odd/even pages. there's a "scale" option for \copypages but it's somewhat naive to use in such practical cases... Yue Wang