Unfortunately not.

Your word usage is a monthly quota: every month your quota will automatically renew, and any words that you haven’t used won’t roll over to the next month.

They are only valid for individual months and you can’t accumulate them. If you didn't use all your credits from your monthly quota or obtained through 'Earn extra credits' actions, those unused credits expire when your billing cycle ends - they don't roll over to your next month and they don't accumulate with your renewed monthly quota.