Sorry for the delay.
I was just looking at something like this. My Intensity Slider product uses a modal for the Add Slide which is called from a non-modal Manage Slides page. Recently a customer had a skin problem that didn't allow for showing the Manage slide so I made it a modal. It seems to work fine when opening Manage Slide as a modal and then the Add Slide as a modal which replaces inside the existing modal.
Here is the call for the Manage Slide:
javascript:dnnModal.show('http://localhost:42477/GettingStarted/tabid/55/ctl/ManageSlides/mid/435/Default.aspx?popUp=true',/*showReturn*/true,550,950,true,''));
And for the Add Slide called from the Manage Slide page:
javascript:dnnModal.show('http://localhost:42477/GettingStarted/tabid/55/ctl/EditSlide/mid/435/Default.aspx?popUp=true',/*showReturn*/false,550,950,true,'');