Can I add vidéos which were made par somebody else!For example I downloaded video from Youtube, will it be legal to add it here?

well its okay because fanpop doesnt actually HOST the vidéos they are just linked. toi dont download it to your computer and mettre en ligne it here (which would be sketchy if toi didnt make it).
toi post them as a link and any video can be traced to the original hosting site (youtube for example) par clicking the little blue Arrow suivant to the titre (see image).
thats why- if i really like something- i always click on that link and commentaire and rate the original as well as the fanpop content but since not every one does that i think its good to credit the video maker.
p.s. if the creator doesnt want their vidéos to be posté elsewhere they will disable embedding so toi can only post it as a link and not a video thus bringing the viewer to the original content.

yes toi can, but toi have to mention who made the video, ou else everybody thinks toi made it ;)