Everyone can add new things in the PUA, in fact, that's where Emoji were before they were encoded. There is the ConScript Unicode Registry [1], an inofficial effort to map various constructed scripts to the PUA without them clashing. There are lots of pages in Klingon out there that make use of that encoding even though it's not in Unicode. Some of those have been encoded formally by now, so there is a way of creating new characters. It just takes a little longer.
Also, by now with webfonts you can easily create your own font to include characters from the PUA you designed, so you can use them online.
Also, by now with webfonts you can easily create your own font to include characters from the PUA you designed, so you can use them online.
[1] http://www.evertype.com/standards/csur/