Convert 10Feb1987 to 1987-02-10 using SSIS expressions - please help!

Hi,

    I have a text file source which has column 'BirthDate'. The values are in the format 10Feb1987. It has NULL values also.
Now, I have to load this values into a Table -column 'BirthDate' and its datatype is DateTime.

I need to convert 10Feb1987 to 1987-02-10 and load the data. If the value is NULL, it has to be NULL (not any default date value).

In the source, I should not select the Retain NULL values checkbox because of some other business reasons.

Can you please give me your inputs as I am a beginner in SSIS?

Thanks in advance!