I wish to get an addon domain, and my present plan does not include addon domains. (Present plan is "SuperProfi") Can I pay to get an add on domain? If so, how much? Or do I have to upgrade to a diferent plan? If so, how much?

kirby.
domain:kirbsand.com

You should pay diference between SuperProfi and SuperSpecial.
In this case you should contact our billing departmenr (billing@hostony.com)