Koha does support EDI, but only EDIFACT. The good news is that EDIFACT is the world-wide standard, with X12 being used mostly in the US afaik. Even given that I haven't found a vendor yet that doesn't accept EDIFACT. Koha 16.05 is the first release that adds EDI support to Koha. Nicole is working on adding it to the manual, but if you need any guidance I can help. If you have a vendor that needs your EDIFACT messages to be customized beyond what Koha sends by default, you can install an EDI plugin to customize the EDIFACT messages on a per vendor basis! You can see the plugins I've written here: https://github.com/bywatersolutions/koha-plugin-edifact-enhanced/branches