Slic3r - Can I add new infill patterns?
Although the core of Slic3r is written in C++ a lot of other components such as infill generation are written in Perl. Since Slic3r is open source there is nothing stopping you from writing your own infill module in Perl and recompiling your own version of Slic3r.

If your considering doing this you can find the existing infill code in the Github repository at

https://github.com/alexrj/Slic3r/tree/master/lib/Slic3r/Fill

Instructions on how to run from source can be found at

http://slic3r.org/download

Here is a commit where a new infill pattern is added. This should show you all the parts of the codebase that must be modified to add a new infill model (some trivial C++ modifications appear to be necessary in addition to the Perl code)

https://github.com/alexrj/Slic3r/commit/2811af349ae17230a525ddcf819b1ddffaa250c9