couple of different way. since I am familiar with the gsk I would suggest using the IBM supplied global security kit or gsk. Not sure what version you are but the command is gsk4ikm (replace the 4 with your version number, could be 5, 6 or 7) . How to use the gsk is a bit much for this thread but the websphere dos should have enough info. you can export everything you nedd from there.
Actually if you got the key from a CA (Thawte or Verisign) you already have most of what you need. It should be a file labeled something.cer.