To clarify a bit further for people starting out with Dexterity, There's no concept of 'transient', computed or calculated fields in Dexterity, so you can't add these to a schema. The idea is that since we use an object oriented programming language the function/method as listed by zopyx and hvelarde is added to the Class that is the python defintion of every object you create as this Dexterity content type.
When you start out with Dexterity the focus is on defining the fields and how to 'get this schema into Plone', either through the web, in XML-schema or in python schema. In the FTI (Factory Type Information) for a content type (CT) this field schema, the views on that CT and other properties like the mentionned class are bundled.
If work your way through the Plone Training documentation on training.plone.org (pick the dexterity chapters if you're in a hurry) you'll see that in chapter 15 in the FTI the klass property of the conent type is set to plone.dexterity.content.Container which is the base class from which dextery CT's inherit.
If you want to add more to this base class for your own CT, you create a new class, MyContentType, inherit from the base class, add the method for your calculated field as a property and you switch the klass property in the FTI to your own class.
I realise this still is very terse in writing and I skipped a lot, but hopefully it gives other people starting out a bit more context and reference to where to start reading/learning more.