Yes, I think it would be good to encourage Google+ users to ask questions here on Stack Exchange instead. For two reasons:
- Stack Exchange is a much better platform for Q&A and findable content (and I don't think I am alone in this sentiment). G+ will just dilute the knowledge base.
- The Stack Exchange beta site needs participants if it is going to thrive. See the statistics on users for this site in Area 51 to get an idea what's needed.
That said, Stack Exchange isn't a good fit for every type of question (see What types of questions should I avoid asking?), so having a venue for asking and discussing more open-ended, opinion-based, or poll-type questions would be beneficial. Maybe G+ fills that role for now. And then any users asking answerable technical questions on G+ can be encouraged to re-post those on Stack Exchange.
As for how to move the question from G+ to SE, I think it best to ask the user to re-post the question on their own (rather than re-post it for them). This way the user base grows and they get a chance to become familiar with Stack Exchange if they aren't already.