I hope this works for you.
First you have to make the stamp in fotoshop/paint/gimp. Save it in .bmp, .jpg, .jpeg,.tif, .wmf, .png, or .psd. I downloaded mine of the internet.
In the drawing environment go insert -> picture... and find the picture you just made/downloaded. Upload it and find a good spot for it.
If you want the stamp in the background of every drawing, you can insert the picture while in Edit Sheet Format mode and save it as a Drawing Template.