Yes, modifying the "Notes" field of a non-complete job is a feature added for Qube 5.3, that was released earlier this week. It can be specified through the GUI with Rt-Click->Modify, through the command line with "qbmodify --mnotes", and through the python API with qb.modify().
Note that presently it will only allow one to modify the notes of a job that is in an incomplete state (pending, running, blocked).