A monthly plan can be added to or upgraded at any point by going to the Subscriptions tab in My Account (available from the upper right navigation when you are logged in to your account or from the large tile on the home page).
Click the Upgrade button and select a different plan. You will be credited for the time remaining in your current billing cycle on a pro-rated basis. Unused pages from your existing plan will carry over to the new plan.
To add blocks, click the Add button and select a block to add to your existing subscription or block.