Hi,
We are currently evaluating eigen3 to port our applications to this library.

We make use of quaternions and we would need to add a quaternion time derivative method.

I looked for the plugin system equivalent to the one in MatrixBase without success.

What would be the best approach to add that feature?

Thank you for your help.

indeed, the few lines of coded required to extend the Quaternion API was missing. So you have to get the latest version from the repo, and then do exactly as explain there:
http://eigen.tuxfamily.org/dox-devel/To ... MatrixBase
for MatrixBase, but using the
EIGEN_QUATERNIONBASE_PLUGIN
macro to specify your plugin file.

Note that this mechanism is to add methods to QuaternionBase (and consequently yo Quaternion), but you are also free to define your own global functions...