Can I assign pre order status to a category in bulk in magento?

It’s possible to complete this action from Magento product grid provided you have an option to sort products by category (this feature is missing in Magento out-of-the-box but can be added through customizations or extensions like Extended Product Grid).

In order to set products from a particular category to Pre order, you need to apply a mass action called “Update Attributes”.  On the inventory tab you will need to choose “Allow Pre-Orders” in the backorders field: