Variable to create new dimension value in Webi

hi There

in a webi report, i have couple of variables, what i need to do is to create a new dimension by these variables, so i can use it in a chart

for example:

i have current month, previous month, previous of previous month, all of them are variables, i need to create a new dimension, so i can use it in a crosstab to create a chart

thanks

Cleo