Dear Vadim,
in measured data, the origin is defined by the hardware and image reconstruction parameters of the measurement device. MR scanners usually set the origin to the center of the magnet. If you position the head of your subject in a standard head coil and position this coil correctly in the scanner, images should actually be quite close to what MNI/Talairach define as origin.
If it is not absolutely necessary, I would not mess around with this. Unless your data are really far off the origin, SPM does a pretty good job moving your data to the correct location during spatial normalisation or segmentation.
Best regards.
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