Hi guys, I just started playing with Poser, and I am curious to know if a certain bone structure can be saved and used with other similar figures. So that I should not start to bone the figure from scratch and instead, with minimal tweaking, adapt an existing one.

Thanks for any help !

Yes, and in fact this is the basis for nearly the entire Poser content market.  You can model clothing for popular figures like the DAZ people and use their rig setup (their bones) to rig the clothing as a conformer to the character.

You can bring in an existing skeleton by loading the CR2 when in the Setup room.
Unfortunately doing so loads not only the skeleton, but the morph channels (and data) which probably won't be useful, so it's best to create a stripped down copy using a text editor beforehand.

You must make sure that the group names match the bone names (again it may save some pain if you do this using a text editor as Poser sometimes glitches when you rename bones).

Well, minimal tweaking is a very subjective term...especially when it comes to rigging, bones and the setup room.
Often times you can spend as much time tweaking something as setting up your own custom rig, of course this will be dependant upon your figure/creature/model that you are working with. The more complex and/or different your figure is the more you will want to create your own rig for it (rig = bones).
If you are going to sell this then you need to be careful what CR2's you are using in the setup room, additional editing to the CR2 may be needed to conform to EULA laws.

There are some helpful tutorials out there that address this issue. I also have some video tutorials that cover content creation over at Content Paradise, they may be above a "newbie" level but are quite informative none the less.
http://dev.contentparadise.com/search.aspx?searchText=Dark%20Edge%20Design%20&catid=6&cat=Books%2c+Tutorials%2c+DVDs

Welcome to the world of creating!
Regards

Thank you very much for all your kind and interesting answers ! I'll most likely stick around this forum and bother you again :-)