Hi Josue,
When it comes to ‘adding’ tracks, this is as simple as running tckedit and providing multiple input track files; they will all be concatenated together into a single output track file.
The harder problem is ‘averaging’ tracks. I refer to this as generating an ‘exemplar’ streamline from a set of streamlines: a single trajectory that approximately follows the centre of the bundle. This was raised quite some time ago in this thread. While there hasn’t been any explicit progress in MRtrix3 here, command connectome2tck does have the -exemplars option, which performs this sort of calculation only in the process of isolating streamlines belonging to individual edges of the connectome, which then interfaces with the connectome tool in mrview. It’s theoretically possible to provide similar functionality for any arbitrary set of input tracks, which I raised briefly in this GitHub issue. If there’s sufficient demand for it, I can add it to the list of requested features. Unfortunately it’s actually a deceptively difficult thing to do for arbitrary track inputs, and naive algorithms for such in my experience tend to produce unexpected results (I’d need to go rummaging through old code I wrote during my PhD to demonstrate it).
Rob