Hi Rafe,
I've seen your great posts elsewhere so thought I might respond to this.
Drilling to your Q, what we've done is use the source version of forums to find the control panel where posts are added. You will find code snippets you can call in your own module code. We've done this for adding groups and forums so I assume it is workable if you just want to add posts to existing forums.
More generally, you might want to evaluate the core forums for your need first. It has limitations that made us go to a commercial option (no vested interest). The latter is much faster and the APIs are clean and easy to use, even if tech support is sometimes intermittent and not always available while they are coding new module improvements. It also has a bunch of additional features.
Spend some time using the forums for DNN itself. Perhaps you will find as we did: 1) it often times out on queries and is very slow when it does respond. 2) it doesn't really support search in any normal sense but ony finds hits that have the query words adjacent, i.e. it's not the normal information retrieval you would expect. 3) the architecture might tend to bog down, insofar as functionality which should be modularized separately (messages and profiles) is implemented inside forums in ways that reduplicate what is available in DNN outside forums. The bug reports we see on core forums is also a concern but we didn't hit anything big ourselves.
Finally, when working with core forums as a developer we found very long delays operationally as our routines called the forums routines. It slows down dev quite q bit compared to the commercial forums.
But beware the commercial forums do not provide source code so you run into real risks and documentation impediments in that regard.
Other things being equal it might be best to support the core forums, but we are wondering if the architecture of core forums might slowly but surely be bogging down. We haven't really evaluated this but it seems superficially it might be so.