I spend some time on it (currently for unindexed triangle lists) and it is quite easy.
var wireframe_pos = new Float32Array( positionArray.length * 2 );
for( var ii = 0; ii < positionArray.length; ii += 9 ) {
wireframe_pos[ ii * 2 ] = positionArray[ ii ];
wireframe_pos[ ii * 2 + 1 ] = positionArray[ ii + 1 ];
wireframe_pos[ ii * 2 + 2 ] = positionArray[ ii + 2 ];
wireframe_pos[ ii * 2 + 3 ] = positionArray[ ii + 3 ];
wireframe_pos[ ii * 2 + 4 ] = positionArray[ ii + 4 ];
wireframe_pos[ ii * 2 + 5 ] = positionArray[ ii + 5 ];
wireframe_pos[ ii * 2 + 6 ] = positionArray[ ii + 3 ];
wireframe_pos[ ii * 2 + 7 ] = positionArray[ ii + 4 ];
wireframe_pos[ ii * 2 + 8 ] = positionArray[ ii + 5 ];
wireframe_pos[ ii * 2 + 9 ] = positionArray[ ii + 6 ];
wireframe_pos[ ii * 2 + 10 ] = positionArray[ ii + 7 ];
wireframe_pos[ ii * 2 + 11 ] = positionArray[ ii + 8 ];
wireframe_pos[ ii * 2 + 12 ] = positionArray[ ii + 6 ];
wireframe_pos[ ii * 2 + 13 ] = positionArray[ ii + 7 ];
wireframe_pos[ ii * 2 + 14 ] = positionArray[ ii + 8 ];
wireframe_pos[ ii * 2 + 15 ] = positionArray[ ii ];
wireframe_pos[ ii * 2 + 16 ] = positionArray[ ii + 1 ];
wireframe_pos[ ii * 2 + 17 ] = positionArray[ ii + 2 ];
}
attributes[ "position" ] = { itemSize: 3, array: wireframe_pos, numItems: wireframe_pos.length };
Though to be honest, I am not entirely happy with this solution, since the position array is twice the size of the original. It feels too heavyweight. Any suggestions for a better approach?