Is it ok if I copy some of the good unanswered questions asked here and ask them in an Arabic Islamic site that has scholars answering and then post the answer here. Would that be considered as plagiarism?
I'm thinking if you need to ask if it's appropriate, don't do it. There's some issues:
If quotes are attributed and references are given, there's no risk of plagiarism (but there's other concerns).
Even so, the community has largely agreed that mostly copy/paste answers are undesirable and subject to deletion (see: Do we want cut-and-paste answers?). We need original content:
We don't want to become a repository of quotes from elsewhere. -- Manishearth, 2012
We want to attract experts to come to Islam.SE and answer questions. By reposting questions on other sites, it detracts from this goal. (Why would someone come here when they could go there and get an answer directly?)
There's copyright and etiquette matters at both ends.
The closest precedent I've seen to what's described is at Skeptics.SE. In some cases, a claim is made e.g. about a scientist's research, and the author of the answer contacted the scientist directly, then reported back on what they said. But this is not simply copy/pasting the question to another forum, then copy/pasting the answer back.