there is a way to do anything, and although i do code in php i've never seen nor used '+=' and rather than look it up i'm gona guess that it just adds 6 to the existing var?
if so /inc %var 6
is your answer
Note: you can also /dec .. type /help /inc or /help /dec for more info
if it was a typo and u ment .= .. i do know that in php, and mirc = of that would be as doing %var = %var $+ 6
or %var = $+(%var,6)