yes you can:
- create an end mark
- put it in the scrapbook
- when you need an end mark, pull it from the scrapbook
- edit > cut
- enter the edit mode of the text frame where you need it
- paste
you now have an inline item that behaves like a character.
(i'm not sure that this works with scribus 1.4, though)
btw, creating a true type font is probably also a good option. nowadays there are simple and free font editors...
ciao
a.l.e