r.patch
r.patch - Creates a composite raster layer by using one (or more) layer(s) to fill in areas of "no data" in another map layer.
Raster (r.*)
ParameterMultipleInput|input|Raster layers to be patched together|3|False
ParameterBoolean|-z|Use zero (0) for transparency instead of NULL|False
OutputRaster|output|Result