Blend modes are mostly simple equations which are run on the pixel colour/ luminosity values for a couple of sets.
So its been a while since I have written shaders but I would assume that if the one you want is missing you would want to write a fragment/ pixel shader that takes the two textures and does what ever blending equation you need.
i.e. piping the pixel colour information from one images pixle (i.e. the first pixle at x:0,y:0) into one part of the equation and the same pixels colour information of the other image into the other part.
I have a list of them in my compositing book but I cant find an exhaustive list on the internet I am sure with a little bit of searching you could find any you need by looking for that specific one. You could use photoshop of equivalent to browse the different types of blend modes.