Only way to affect the natures of the eevees, to my knowledge, is to give one of the parents an everstone at which point it will increase the chance of that parent's nature being passed on by 50%. but as you don't have a modest/timid parent, i assume, that is of no help to you.
I thought maybe there was a guide on which two natures are more likely to end up with a particular result. I guess I'm going to have to keep on breeding until I get what I want unless I can figure out the PokeTransfer.
It's true. In BW2, everstone passes on the nature 100% of the time.
My best suggestion is to find a male pokemon (or ditto) in the field group that has Timid or Modest, give them the everstone, and then breed. Otherwise, just breed several eevee's. Eventually you are bound to get one with the nature you desire.