If you also want to shift the time points of the various slices to be in alignment with one another, then you could do this with 3dTshift's -tzero option.
If you don't want to shift the slices separately, then you have 2 options:
(a) get one of us to modify 3dTshift to do exactly what you want
(b) you can easily do linear interpolation just by averaging the neighboring (in time) T values with 3dcalc; or by using 4 neighboring values do cubic interpolation
bob cox