>>326269640
You can combine the meshes in Blender by selecting both of them and using the keyboard shortcut "ctrl + J".
You will have to merge the armatures after and reparent the bones if there are any extras (parent drill hair bones to original body's head, for your example).

 If the addition has different names for the same vertex groups (head_00 instead of Head, for example), you will have to rename the bones to the same as the original before merging meshes.