Add Adjuster after Order is completed?
At the moment, both are not possible. The order is locked from recalculation when the order is marked as completed - the final price being locked in.

In a future release we will introduce the concept of order editing after order completion.

Right now, you could manually insert an adjustment row into the database, along with the changes to the order's order.baseShippingCost but you would need to make sure the order.totalPrice and other calculated fields would also be updated in the database to make the order internally consistent.