Thanks. But after transferring skin weight and converting it to accessory the add to custom (+) is grayed out.
You should not convert it to accessory. That is effectively wipes out all transfers and assign 100% influence to the head.
You should keep it as cloth. After transferring skin weights, custom skin weights assignment is necessary.
1. Assign 100% weight for all mask vertices to Head bone (the value of 1).
2. Paint mask for the JawRoot bone approximately as shown on screenshot.
This would allow bottom part of the mask to move as person speaks up.
