how can i extend engine/shopware/core/sExport.php in my plugin?

I have an additional field attr21 in table s_articles_attributes.

In order to export it, i need to add this field to sCreateSql() function inside engine/shopware/core/sExport.php as follows

              $sql=" ......

                d.weight,
                d.position,

                at.attr1, at.attr2, at.attr3, at.attr4, at.attr5, at.attr6, at.attr7, at.attr8, at.attr9, at.attr10,
                at.attr11, at.attr12, at.attr13, at.attr14, at.attr15, at.attr16, at.attr17, at.attr18, at.attr19, at.attr20, at.attr21,

                s.name as supplier,
                u.unit

                ........";

But the problem is i cant add it directly over here, so I need to extend sExport.php class and add attr21 through my plugin?

how can i extend sExport.php?