Convert soft selection to skin weights. This tool is part of the rjSkinningTools.
Copy the rjSkinningTools folder to your Maya scripts directory:
Convert soft selection into skin weights. More influences can be added by clicking the plus button. When more influences are added these weights are automatically blended. The influence and soft selection can be reselected by right clicking the influence widget and using the context menu.
Skin cluster settings like max influences and normalization will be taken into account when applying the skin weights.