How to generate key points by myself

Hi, i want to compute SURF descriptors on my own key points. The problem is, currently I only have 2d image points (x,y-coordinates), but opencv's method for calculating SURF descriptors needs keypoints, which need beside the coordinates additional information like, scale, size, orientation,... (information which I do not have). Please help -> How can I generate correct Keypoints out of my 2d image points? (Correct means, I want to have key points on exactly the coordinates of my image points). Thanks in advance Patricia