Brice,
I uploaded a simple TML to this forum (named "Sample TML DrillPlanExport.zip"). The macro will take selected drill plan holes and write some data to a file. If you use the ViewProjectData macro, you can examine how drill plans are stored in the memory model of a TBC project.
Also, you might use the Visual Studio object browser tool to examine all the properties of a drill hole object. Drill holes are defined in the Trimble.Vce.Alignment assembly.
Good luck..