1) What are main step and sequence: (es. 1-Propose Topic 2-Attract expert 2-collect question
2) What are main tools to recruit Subject Matter Expert (channel to contact, incentives, ....)
I would suggest setting up a LinkedIn Group and then sending out individual inMails to experts. You want to craft your inMail in such a way so they'll want to participate in the group. You should be able to find the right experts by leveraging your own network or using filters to find the people you need. Make sure you have some running threads/topics in the group. If there is nothing to talk about your new found expert will quickly walk away.
LinkedIn can be a great way to find people. But building on LinkedIn is like building on quicksand. Your goal should be to build your own platform / venue and bring the conversation there.
Social media has its uses. But you should build your own business, your own brand -- not theirs. Ultimately LinkedIn or Facebook or Google Plus owns your members. They own your written content too.
LinkedIn in particular has some potentially fatal issues, to which management appears to be indifferent. Content is frequently deleted with no opportunity of recovery. Group managers (and now, it seems, even group members) can sabotage rivals -- in a single click preventing them from posting across all LinkedIn groups. With risks like these, you should build on your own soil.
Blogs on niche topics take a lot of legwork to grow at the beginning, especially if you don't have an already-existing community as a starting point. Your best bet is to research who blogs on these topics already, and ask them to contribute to your blog. You'll likely want small-to-medium-sized bloggers, not the big shots, because they'll appreciate the opportunity.
By going this route, you'll find people who both 1. know how to write and blog and 2. know the niche inside and out. If you just look for people who know the niche, chances are they won't know how to write blog posts that are interesting to read.
Give us a shout if you need more help (this is our specialty): http://socialexis.com. Good luck!