Hi Carlos,
I assume that EV1 and EV2 are 1st level EVs (otherwise you wouldn't want the interaction). Then you can compute the interaction in a new model as:
newEV1 = oldEV1 - oldEV2
newEV2 = oldEV1 + oldEV2
newEV3 = RT
newEV4 = newEV1*newEV3
newEV5, etc = other EVs you may have
Then create a contrast that tests newEV4:
C1: [0 0 0 1 ...]
C2: [0 0 0 -1 ...]
Note, however, that the oldEV1 and oldEV2 need to have already been convolved with the HRF. Perhaps the best way is to simply copy them from the already generated design matrix from FEAT, then create a new model in which these already convolved EVs are entered (but don't convolve again then).
Hope this helps.
All the best,
Anderson