You are correct. There is no way, out of the box, to do that now.
I created an issue for this here that will target version 3.0.1:https://github.com/BroadleafCommerce/Br ... issues/193
In the mean time, you can create a new endpoint yourself that allows you to do this. You should likely add a method to your fulfillment endpoint for this. Essentially, you will need to find the FulfillmentOption by id (according to the one you pass in the request), and set it on the fulfillment group. You will also want to set the fulfillment type on the fulfillment group. It should, as a general rule, be set to the same fulfillment type as the fulfillment option's fulfillment type. The reason that we store fulfillment type on the fulfillment group, in addition to having it on the fulfillment option, is that before you know which fulfillment option the customer wants, you may need to group the items according to type. This should be quite easy to implement as a custom extension.