Okay, I've downloaded the pack, and had a look.
If for example you wanted to have sandstone share a tile with Diorite.
In the "Grim Dark" folder, SSGD_igneous_intrusive.xml contains 3 parts that say "<subtype value="DIORITE"/>".
Directly after those you can add "<subtype value="SANDSTONE"/>".
If you turn on Debug Mode for stonesense, by changing "[DEBUGMODE:NO]" to "[DEBUGMODE:YES]" in the init.txt file within the stonesense directory, a tile you select in df, with "k" for example, will have info in the bottom left corner of stonesense. Most of the time, the ID name is the same as ingame, barring being all capitals, otherwise you can see here what you need to add to <subtype value=""> to get it to include this.
I don't have a list of sedementary stones on hand, unfortunately, so you will have to type them all out; unless someone else has them.
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On block walls, I haven't found any entries describing them in grimdark, you can make your own entry though.
Here are settings for stone block walls pulled from my own inprogress (unreleased) pack.
<block sprite="0">
<terrain value="495"/>
<terrain value="496"/>
<terrain value="497"/>
<terrain value="498"/>
<terrain value="499"/>
<terrain value="500"/>
<terrain value="501"/>
<terrain value="502"/>
<terrain value="503"/>
<terrain value="504"/>
<terrain value="505"/>
<terrain value="506"/>
<terrain value="507"/>
<terrain value="508"/>
<terrain value="509"/>
<terrain value="510"/>
<terrain value="511"/>
<terrain value="512"/>
<terrain value="513"/>
<terrain value="514"/>
<form value="block" />
<material value="Inorganic" />
</block>
Change the sprite on the first line to the image you want.
If you want different pictures for different stones, make multiple copies with <subtype value=""/>'s.
If you want it tinted to the colour of the stone, use a gray/white stone, and add color="material" next to sprite="".
Numbers for the sprites start at 0 on the first line 20 on the second line and so on, regardless of how wide the spritesheet is.
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I think I've covered everything, let me know if i missed anything.
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Also @ japa, if you can add so doors are considered walls in border_wall_NOR, border_open_NOR and the like, It would be appreciated(If you have the time).