You should be able to use the subnav code above to add a “My Topics” subnav to the Forums tab. I’ve done it with one of the other existing tabs. You just have to find the bp_core_new_nav_item() call that adds the forums tab. Then, use some of the code above as a model to add a subnav I think.
The other key is that the ’screen_function’ is the name of the function that will be called when that Nav is clicked.