Update : in my harmonic analysis, the previous modal analysis is restarted with the command ANTYPE, MODAL, RESTART (as shown in the ds.dat file). During this restart, my resvec command is taken into account : 

antype,modal,restart    ! restarting the modal analysis
modcontrol,on,on ! enforced motion turned on
mxpand,,,,yes,,yes     ! expand requested results and write them to file.mode

it probably means that some pseudo mode shape is calculated during this restart, right ? and then stored in the mode file of the modal analysis ?