For FW 5.20 (i.e. not your case, but likely to apply with minor modifications) the answer is yes to all three. Off the head (if relevant I can look up the details later):
- the url is in some /mnt/ext1/system/config/ file
- there are some, I think I'm using the Korol'ev among others. At most you have to convert them from some format like stardict to .dic. Here is one source: http://www.libredict.org/en/
- after rooting easily with samba, but within KOReader should be possible as well (e.g. ftp). There may exist also some older utiliy (vlasovsoft suite?) allowing that too.