Is it possible to mod Eador: Genesis? And if so, how?

http://www.gog.com/forum/eador_series/moddingindexes

Mostly through editing var files with Notepad.

As Gremlion points out (he's always beating me to the punch), a large fraction of the game mechanics are laid out "in plain sight" in the var files. For example, unit.var contains the attributes for all the game's units. With a bit more work, it's also possible to add new art in the dat files.

For an example of what modders can do, take a look at the "New Horizons" mod (which I help translate to English)
http://eador.com/B2/viewtopic.php?t=3520

Basically, they added MORE of everything: units, spells, rituals, races, items, events, etc
http://eador.com/B2/viewtopic.php?t=3044

(Some of their changes did require source code access though.)

What are you thinking of specifically?

Regards.

I changed some graphics thanks to Gremlions thread there, its pretty easy with grabber.exe.