This can be done, but it depends on exactly what you're trying to accomplish. If you're trying to have users upload something into a bucket that you own, you can do so using post uploads with a simple HTML form. You can use the documentation here and substitute .objects.dreamhost.com instead of AWS - http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingHTTPPOST.html
If you're looking for the ability for users to upload to their own buckets that they own, then they'd need a DreamObjects account. We have a partner API that's currently in testing. It will allow the app to retrieve a DreamObjects bucket and credentials and pass them back securely with the user having only to login (or create a new account if they don't yet have one).
The first method is fully functional now while the partner API is currently in testing.