After reading and trying some more, this works:

code:
TextField.prototype.addProperty('_bold', this.getBold, this.setBold);



code:
function getBold(){ 

   format = this.getTextFormat();

   return format.bold;

}

function setBold(bool){

   format = new TextFormat();

   format.bold = bool;

   return this.setTextFormat(format);

}