Yes, you can. You need to go to your servelet's control panel in our customer portal, and click on the "Upgrade" button. You will then be able to see the option to upgrade just your disk space with the slider under "Resource upgrades", in addition to upgrade/downgrade your plan under "Upgrade package" (see screenshot below).
Select the new disk size that you would like to have, and then click on "Continue". The cost for the additional disk space will be automatically calculated accordingly.
Plan XS and Plan S servelets created before 22 January 2014 might not be able to see the "Resource upgrades" and the slider. If you would like to upgrade and need the disk size slider to be available for you to upgrade, please open up a support ticket with us.