how to format the date in string format to specific date format from the background code

Hi, I have a column stored date in varchar type since user want to input N/A if there is no date available. If the date is available, it is in this format 05/02/2015 but it is actually a string.  
how could i convert the date in varchar type to this format dd-MMM-yyyy. I don't want to change the column datatype to date because user will not able to input character like "N/A" in there.
Thanks.