The invention relates to a method of modifying data in an encoded data signal 125 corresponding to successive pictures divided into sub-pictures, for providing an output modified data signal 126. In particular, this invention can be used for the insertion of an additional data signal 127 into a compressed video signal 125. The proposed schemes according to the invention are based on a transcoder arrangement including at least partial decoding means and partial re-encoding means. This method leads to a cost-effective solution compared to the prior art comprising a minimum number of functional sub-steps, in particular including a unique motion compensation sub-step 205, taking advantage of simplifications and combinations between different sub-steps.