Hey Yannick,
You can do this from the Report Options dialog. I actually posted something a while back about doing this. See here: http://forums.ni.com/t5/NI-TestStand/TestStand-2012-Change-the-UUT-data-type-now-contains-a-new-quot/m-p/2393110#M40635
I hope that helps, and let us know if you run into any more questions about it!