Is it possible for users to add new Online Photo sharing websites to the Digikam "Export" list?
Sites like http://www.ipernity.com and http://500px.com/ are popular but not listed?
Or does this need to be a feature request?

Also how can I remove Photo sharing sites from the Digikam Export list? There are so many that it is difficult to find those I use regularly.

You need to file a feature request i'm afraid.
Uploading to photo sharing sites is accomplished using a mechanism known as "Kipi" which also allows for a number of other actions to take place (and provides all the other actions available in the same menu you find the photo sharing site entries).

Oh. What a shame.
Thanks. I'll do that.