Here are the steps
Pose the rig so that 2 or more poses are activated at 100%. Their weight value is 1.
Create a combo pose
If you click on the combo pose a custom ui should be visible that lets you load in poses.
Load in the poses that make up the combo through this ui.
Their weight values in the ui should be 1
If their weight values are not 1, repose the character and right click on the combo pose and Update Pose (Controls Only) ...
If all the poses are at 1 than you are good to start sculpting. Should work now like any other pose.