ImBatch from High Motion Software https://www.highmotionsoftware.com/products/imbatch can extract channels, and will open DDS images - although I'm not sure I have any to test. The interface is kinda clonky, but it's free for non-commercial use so it wouldn't hurt to try it out. I keep it around for the occasional task that IrfanView et al can't handle - it's very powerful once you've got to grips with it.