The contour of the ideal female forehead is smooth and rounded, whereas the ideal male forehead typically has a broad ridge above the brows. The short answer to your question is that fillers can be used to fill the indentations and create the smooth forehead. However, this should be only be performed by experienced injectors, since, as you suggest in your question, there are blood vessel danger zones in the forehead that must be avoided. Hyaluronic acid fillers are ideal for this purpose since they can be dissolved if necessitated by hyaluronidase. Permanent fillers of all kinds, including fat, should be avoided, since these are difficult to impossible to remove easily and no enzyme exists to dissolve them. Make sure you consult with a board certified aesthetic physician. Best of luck.