iirc, scribus uses libreoffice dictionnaries for hyphenation... so you might hack yourself one on top of an existing dictionnairy and use a custom language...
but that's a hack...
being able to feed custom hyphenation files would be a good feature: do you know any (free) software that allows it? if possible while using hunspell?
and for preventing hyphenation, the only standard i know off, is to put a smart hyphen in front of the word. this will prevent the word from being hyphenated.
i've dont think i've ever seen a way to avoid the hyphenation at a specific point in the word. do you have an example?
and for the general case, i think i recall having seen references to it and patches in the bug tracker but i don't know what's the current state. it might come with the "complex text layout" (arabic) integration.... hopefully in the next few months!