yay, it works!
somehow..
now i can load the Spell.dbc an look at the content, but how does adding new entries work? there's only a "copy selected entry" button, that doesn't seem to work.. :/
ps: if adding is possible, is it also possible "line-wise" copying csv-formatted entries?
edit: the working spell.dbc was convertable with dbcutil, but after merging my "fix" in and re-dbc'ing it was first of all SMALLER than the original dbc (formatting error - dbcutil? outdated?) and also loading the smaller file, the server crashed permanently after characterlogin.. seems arcemu doesn't like dbc-injections at all