For WarCraft III models by Blizzard? No you do not need permission, although you cannot claim credit for the conversion (since what you are doing is technically not legal) and must credit Blizzard with all rights for the model (they own distribution copyright over the model). It still technically violates the game Terms of Use so Blizzard reserves the right to remove and punish you as they feel appropriate (currently they act as if they do not mind but they might take a different stance in the future).
For WarCraft III models made by a third party? You do not need permission either as long as you have the necessary copyright (which you do for models in the public domain like those on this site). You can also claim credit for the conversion but must credit all original authors with making the model. This site might have a different view on the subject as they do not necessarily follow UK copyright law.
Be aware that it is highly likely that this site will reject such conversions on the grounds of poor quality. StarCraft II requires models of a considerably higher quality than WarCraft III to blend in with the game. World of WarCraft models do reach some of these quality requirements (mesh detail, animations) but still lack the required texture layers. Without the required texture layers the models will really look poor when viewed at Ultra settings (recommened for custom maps).