You can get the culture from GEM http://www.gemcultures.com
. Natto is pretty simple if you have a heat source, and it might be adaptable to a soup base. But Ive seen how it grows naturally (wild, without a starter) and a liquid substrate might not work the best...seems like it wants whole beans with lots of air pockets in between. You could always make your natto per instructions, then grind/puree it, and add it raw to a soup like you would miso.
Just be aware it is very unique in its flavor/texture, nothing like miso. To those less adventurous with food it may be off-putting. But I like it.