Dasleah, I believe that point has been understood. He's specifically asking for help on how to use a utility in order to provide the desired effect. Also, Dwarf Companion is not fully equipped to handle tile editing, it is better off working on living creatures.
Rick's Tweak utility (which I assume is the one you're using, since I had a similar idea looking through the list of options) can indeed change a tile to sand. It has to do with editing the goegraphic index of biome information or something, I forget what it's called exactly. It's on the same page as the fluid info.
I haven't had much success with terrain changes, however... I keep ending up with one very suspicious-looking tile that continually changes, and that worries me.
There's anothe method, however. Go into the 'raw' folder, find the matgloss_stone_soil.txt, find the entry for the dominant soil type on your map, and replace the [SOIL] tag with [SOIL_SAND]. Apparently, this does not require another world generation.
As for magma rivers, I don't think it's going to be as easy as a simple aquifer switch (which I tried). I'm sure there's a way to recreate a "magma flow" tile like the ones at the bottom of the magma pipes, but I haven't had much progress in that area.