You could use a DDS utility, like the excellent freeware DXTBmp by Martin Wright, to extract the DDS files to a graphics editor of your choice (Paint Shop Pro, Photoshop, etc), edit them, and import them back into FSX.
(I'm sure there may be other DDS converters around but I can only speak of my personal happiness with DXTBmp).
Hope this helps.
**********
Having just tried this method though, it is pretty difficult to do a decent job, it really would be better with source files that are "layered" - either PSD's (for Photoshop) or PSP's (for Paint Shop Pro).
Anyone know if anyone makes them?
For that matter does anyone know of a decent freeware repaint kit? (I see Xload is payware)