Convert NSPoint in screen coordinates to window coordinates

I am implementing drag and drop in a cocoa application and I am using the following method from the NSDraggingSource Protocol:

- (void)draggedImage:(NSImage *)draggedImage movedTo:(NSPoint)screenPoint{

The NSPoint is given in screen coordinates and I need it in window or view coordinates. If the method was called with an NSEvent I could do the following:

    -(void)mouseDown:(NSEvent *)pTheEvent {

 NSPoint tvarMouseInWindow = [pTheEvent locationInWindow];
 NSPoint tvarMouseInView   = [self convertPoint:tvarMouseInWindow fromView:nil]; }

But I cannot figure out how to do it with just an NSPoint. Thanks in advance!