I cannot speak for Nisus, of course, but I think that your request is unlikely to receive any priority.
A workflow that writes equations in an external editor and "inserts" them into Nisus does not appear to be workable. The idea that a document can be divided into equations and text is wrong. Many documents contain inline math in addition to equations. What could be inserted from an external editor is the LaTeX code, but this is plain text, not an image of the equations or symbols. So the Nisus document would become an unreadable combination of nicely formatted text (awaiting conversion) and LaTeX code. If, instead, you use Nisus to convert the "non-math" parts to LaTeX, then insert the "math" LaTeX code into the resulting plain text document, the original Nisus document would also be difficult to read--it would be missing all the math and other symbols that you will add from the equation editor. Also, the piece-by-piece insertion of the external editor's produce would greatly lengthen the process of writing, and you would not know if you had done it correctly until you previewed the results of the final conversion.
Nisus is not well suited to heavy mathematics. Many math and non-math symbols that are easily coded in LaTeX have to be inserted from the Mac's character palette, a laborious and slow operation. I find it much easier to type "\theta" or \copyright" in an expression than to find the corresponding symbols in a symbol palette and insert them into the text. LaTeX also has many "styles" not found in Nisus, for example a "typewriter" style.
Contrary to your impression, there are already document editing systems that convert to LaTeX. On the Mac, try LyX. (http://www.lyx.org
, free) or Publicon from Wolfram. Both have a graphical interface, so that you see formatted text on the screen. The formating of the final print version is determined by the LaTeX specifications. This type of editor is WYSIWYM "What you see is what you mean", compared to a word processor like NWP, where the the screen and print appearances match (WYSIWYG). There are many keyboard shortcuts for entering math. In both you specify the type of text you wish to enter (title, section, list, paragraph). I've used Publicon extensively, but LyX appears to have similar-or better- functionality and, unlike Publicon, is continuously developed.
Although LyX and Publicon convert to standard LaTeX document classes (such as the American Mathematical Society (AMS) class), they don't meet the needs of journals that supply their own document classes and styles for TeX. To write for such journals, you will need to write your own LaTeX code. There are many resources for starting. See: http://mactex-wiki.tug.org/wiki/index.p ... ng_Started
. TexWriter and TeXMaker at Macupdate look like promising systems. I use TeXShop , but this requires for each document a folder that holds auxiliary files; this is workable, but clumsy. A suggestion: preview pieces of LaTeX code in LaTeXiT, rather than run them through a full LaTeX processor.