Elaine is correct that there is no easy command equivalent for this.  I wrote a short Python script to label the middle label of each secondary structure element with α/βN for helices/strands respectively.  Run the script simply by opening it with the open command.  You can adjust the initial height of the labels by editing the first line of the script, or afterward with the "label height N" command.  Keep in mind that secondary structure element numbering resets with each chain, so if you have a multi-chain structure then this script may not produce what you want.