you might not be able to use the stamp in your actual stamp collection, but if you don’t mind fiddling with bbcode you can use the [img]url[/img] tags to add stamps to
your signature :) once you’ve done that you will have to view
your own profile and click the “display under your forum posts” next to the signature section.
not only can you add stamps you don’t have on your account then, but there are also lots of unofficial user-made ones in the
art gallery forum if you’d like to browse around! they’re usually called “buttons” on these threads.