[NTG-context] Duotones with spot colors

Hello list, I am preparing for a next project which will be printed in two colours. Obviously, perfect opportunity to use duotones. However I got my duotone illustrations in CMYK tiffs with images in yellow and black channels. Obviously black could stay but I would like to convert the yellow channel to the spot colour I am going to use. Any ideas (without using Photoshop) ? Many thanks in advance Piotr -- http://okle.pl