My project requires me to convert DITA to nroff, but the toolkit I'm using (DITA Open Toolkit) only converts to troff.
Are there any toolkits (preferably java, but any language will do) that can convert from troff to nroff? I'm not even sure of what the difference is, but from some googleing they appear to be very closely related.
They are really close related. Most format commands are understood by both tools, the main difference is how the final output looks like when you print/view the documents after running it through nroff/troff (see this article for an overview).
So you should get away with the tools you already have. Note that on most modern Unix systems, groff will be installed which comes with both frontends.