This additional tool (available in TTT 2.4 and newer) allows you to incrementally build up a texture covering, or add terrain textures with splatmap control to an existing terrain. You can also use it if you need more than 8 different textures, and don't want to modify the main script.
Using it is extremely simple. Just like in the main tool, you need a splatmap with R,G and B channels representing three different textures. In the dialog window, drag your terrain into the terrain slot and your splatmap in the splatmap slot. Drag the textures for R,G and B into the respective fields, and you can also set the tile size here directly. Note that all three slots are optional, so if you want to add only two textures and they're in the G and B channels, leave R empty.
Note that in this tool, black means no texture and will leave the terrain texture at this point unchanged. You can, of course, use shades of colours to indicate less coverage (so a dark red would add the red texture, but at less strength). The alpha channel is not used.
Finally, there is one slider to set that controls how the new textures are blended with existing textures where they overlap. At the default setting 1.0 the textures will be blended with equal strength. Slide it to the left to give preference to the existing textures, and to the right to give preference to the new texture.
If, for example, you are adding roads, stone paths or fields to an existing terrain, you want it pretty high so the original terrain is completely covered. But if you add foot paths or patches of green to make more variety (say, from a noise texture), you want it more low so the old texture dominates and the new influences the final look only slightly.
Experiment with this slider to get a feeling how it affects the final visuals of your terrain.