Qpixmap - direct swapping only rgb (without alpha) channels

Hi. I wonder if it's possible to swap only rgb channels and use old alpha channel. I have 2 images at input - one rgb IplImage from Opencv and one white Qpixmap with alpha. I need to "draw" cv colors to Qpixmap, but left old alpha channel. I've read somewhere that it's not directly possible, so I'm interested in the most optimal way to do this (it needs to be in real time (up to 10 fps where swapped pixels area is about 300x150 px and Qpixmap is about 360x200) with cpu usage about 5%, maybe less, so If it's really heavy operation I'd think about convert all 276 Qpixmaps to Qimage for direct access, but only if there is no other way.