I wrote a program in Python which you could modify to do that:
http://pastebin.com/Wt3LRDcL
Just save it as a .py file
put the words you need to convert in a text file in the same directory
open up the .py file and change the leet_dic dictionary to what you need... (if you just want those 3 things, remove all the dictionary listings and change it to just {'o': ('0'), 'ch': ('4'), 'sh': ('6')} )



