No (and yes). We are working on an extension system which exposes functionality to create your own shared libraries written in C. Forum user @RG1 has experimented with an alternate non-cross-platform approach: Lua Brigde - Custom Leaderboard?
Note that you can also use Lua modules to share code between projects, but depending on use-case the performance might not be good enough. Did you have a specific use-case in mind?