Append data to a list within a dict

[Tina I]

Hello group,

Say I have the following dictionary:

ListDict = {
'one' : ['oneone' , 'onetwo' , 'onethree'],
'two' : ['twoone' , 'twotwo', 'twothree'],
'three' : ['threeone' , 'threetwo', threethree']}

Now I want to append 'twofour' to the list of the 'two' key but I can't 
figure out how to that?
Some pointers would be greatly appreciated.

Thanks
Tina