mask/rules

I have a hash that has a description of the type of password it is. 
It says "8 lower case alphabetic with the following charsets with the probabilities listed:"

asdfghjkl = .5 probability
ertyuicvbnm = .4 probability
qwzxop = .1 probability.

I understand using a charset, like: -1 asdfghjkl -2 ertyuicvbnm -3 qwzxop 

This leaves me with the question, is there a good way to do this?

Thanks.