Its been answered.
Can't see the solution actually... Could you please post it? Just for everyone else who might by interested in that topic? Thanks!
If your cursor is anywhere in the text, its insertion point is
Inserting a character to its right, for example, can be done using
app.selection.insertionPoints.contents = "!";
-- this works because the Insertion Point is "between" characters.
(I would also have shown you how to replace the next character, but my first try crashed ID.