Due to size and color limitations you can't do it directly, but you can export RMXP graphics then try to modify them to work for RM2K3. I wouldn't really recommend it since there's so much work to be done to make RMXP graphics look decent on RM2K3; you might as well make original graphics. If you do decide to try for original graphics, let me know and I'll send you a few good pixel art sites with tutorials and more to get you started.