Is it weird/creepy if you add a girl (you fell in love with after hanging out a week) on Facebook although you are sure you won't meet again (unless you make a date)?

First off this is just Facebook. Strangers friend other strangers all the time and you two aren't strangers so there really isn't anything wrong with it. Most people these days friend people they know of, even if they aren't actually friends so it isn't all that weird. If she thinks that it is weird or creepy, then she's probably reading too much into it and isn't interested and you'd know that it's time to start looking for someone else. Unless they're reading too much into it, most people don't see friend requests as something unusual or strange; it's only the people doing the friending who think that.

Also there are a lot of worse things that could be considered creepy and weird. For instance, openly stalking them, staring at them all the time, taking secret pictures of them, and stealing their belongings are creepy (while some of them are also illegal). If this girl finds friending her on Facebook on the same level as all of those other things, then do you really think that you want to go out with her?

But also remember that if she doesn't friend you, it doesn't necessarily mean she thinks you're creepy for it. She could be the kind who only friends people she's known for a long time but doesn't find your request as a weird thing. Or maybe she's not interested and doesn't want to give you the wrong idea by accepting but isn't creeped out. Point is that there are many different things that could go in her mind and it is just a waste of time to wonder what those things could be. Just friend her and you'll get the answer to that soon.