You first do some research on where to put the moxa modules (to check where debian puts them would be a good place to start) - it should be somewhere analogous to /usr/local/lib/modules/3.8.10-tinycore/kernel/drivers/tty or similar.
Then you use gzip and advcomp to compress the moxa modules, copy them to the filesystem and make an extension of them - most, if not all, of this is explained in the wiki and I posted a link to how it was done for the wl module extension a few posts back.