There is no admin option to do that, you would need to create a module and hook into native DHL carrier one. But what you can try and do is to set cart price rule that does not require a coupon (so it will be applied automatically) and set it as follows:
In conditions tab
If ALL of these conditions are TRUE :
Subtotal (Excl. Tax) equals or greater than 200
In actions tab make discount amount 0 and under
Apply the rule only to cart items matching the following conditions (leave blank for all items:
Free Shipping: For shipment with matching items
This should switch shipping price to 0 for all shipments. If you have many options in DHL carrier that may not be enough though.