There are a couple of ways I would handle this. 1. Unwrap the first one and then clone the model and modify it as needed. 2. You can try the copy / paste in the quad menu.
Did you modify the geometry after UV mapping? that could be the reason why you're seeing texture stretching. Try UV mapping those stretched sections again.
If you're on Max, simply add a vertex paint modifier and start painting. But for it to work in game, the material you are using needs to have "vertex colors" enable in the shader params.
I just did this recently, the old way still works in Max 2016. Skylight VertexPaint modifier. Radiosity Setup, Start. VertexPaint: Assign. Radiosity: Reset All.
Make sure perspective is off, and perhaps limit which directions your brush pushes in (transform: modifiers). Or try a more subdivided grid with a higher wrap mode setting
So I took about 30 min an used a poly primitive Cube and sketched only with hPolish,dam_Standard, and Clay tubes with a noise modifier and this is what came out.
You may also be interested in the IMM brush. Add your new shape to it, and you can use the Projection Modifier to automatically deform it to a surface while you draw it on
You have the change the material id of the objects faces in the modify tab. To get the image in the editor, you have to load it again. The option should be in the same tab though.