My oun mods

[TippeX]

generally no,any modification that requires modification to the executable (ie: needs cracked exe or whatever) is illegal and breaches the EULA
if the mod is like a data file, and you use a loader to inject a dll or code etc, then its questionable really, but as a general rule of thumb : do not modify exe's permanently

if you distribute modded executables without permission, then its definately illegal...