As Millie says, Decker uses its own font format; adapting e.g. an existing TrueType font for Decker will require some manual work, and perhaps a bit of script-wrangling. Your first stop should probably be fontedit.deck, which includes a very simple font editor. As discussed quite recently, since this editor is a deck, you can feel free to extend and customize it to suit your needs.
I wrote an article some time ago which describes how one could build their own DIY importer for pre-existing bitmapped fonts.
The structure Decker uses to represent fonts is described in The Decker Document Format, but this information is only necessary if you wanted to produce Decker fonts from some external tool. The Font Interface in the Decker Reference Manual describes the APIs for manipulating fonts on the fly from within Decker.