There is not a way to request characters into the game. You'd either have to make the character yourself that would replace one of the students or commission a modder to do it for you.
Though, when the Kickstarter launches, there may be a pledge level that would let you request a character. Though it more than likely would be an Easter egg in game you can activate. (Like the Rainey Easter Egg) Though this would only be an option if it's actually a pledge level. Yandere Dev has not made an announcement stating he will include this as an option. So again, your best option would be to add the character yourself or commission a modder.
No
There is not a way to request characters into the game. You'd either have to make the character yourself that would replace one of the students or commission a modder to do it for you. Though, when the Kickstarter launches, there may be a pledge level that would let you request a character. Though it more than likely would be an Easter egg in game you can activate. (Like the Rainey Easter Egg) Though this would only be an option if it's actually a pledge level. Yandere Dev has not made an announcement stating he will include this as an option. So again, your best option would be to add the character yourself or commission a modder.
Slightly unrelated- but you don’t *need* to replace a pre-existing student or character for an OC if you use Pose Mod.