In the case of a private company, if the articles grant powers to the Board to appoint Directors, the directors can be directly appointed in the Board Meeting and Form 32 can be filed for such Director showing him as "Director".There is no need to first appoint him as additional director and then regularlise him in EGM or AGM.
However as per Section 255, the Directors,in the case of a public limited company or of a private company which is a subsidiary of a public company, shall be appointed by the company in General Meeting. If the directors are appointed in the duly convened EGM / AGM, then they can be directly appointed as Director and no need to appoint them first as additional director and then regularise him.
But the actual practice in most of the companies, to avoid convening EGM, is to appoint such director as additional director under section 260 and then take up his regularisation at the time of AGM.