When to comes to MMO we have to put security over customization. So players won`t be able to mode anything in MMO mode.
We might add a feature to make custom hero and units - but these mods would need to be submitted to a team of reviewers and added with following patches. We might consider creating a team to review user created content for MMO and adding it for everyone to enjoy...we`ll see if this happens.
However, players will be able to install and completely mod a second copy of Dawn of Fantasy not used for MMO or multilayer gameplay.