It's possible, but unfortunately a bit obtuse in this language (you might want to just use photoshop for something like that...).

One way you could do it is as follows:

OPERATE SET R = (IN[COL,ROW-1].R + IN[COL-1,ROW] + ...)/8;

Unfortunately, that's just the R channel. I should find a way to consider COLOR that would be smart enough to just have it apply the operation across all channels. (right now, the COLOR keyword is nice for simple "set color to this" type of things)