Is it possible to create an add in that works on all office versions?
I searched the msdn and different forums but there is nothing specific about this issue.
Difficult if you use VSTO, because you'll need to deploy/assure different runtime libraries. See the VSTO entry in Wikipedia, not to speak of different development and test platforms (which you may already have, if you aim for it).
There's a MSDN Blog from Mary Lee the migration of 2007 to 2010 projects.
Please see also this blog from Andrew Whitechapel: Can you build one add-in for multiple versions of Office?
From a pragmatical point of view ("time-to-market"), you may consider the use of a third-party-tool (www.add-in-express.com), which allows the creation of version-independent add-ins.