In the Step Selection (once you go into a playbook, at the top left), there is ‘Blocks’, which you can use to attach a block to a particular step in a playbook.
Alternatively, in the swagger you can use the endpoint /api/external/v1/playbooks/AttachenestedWorkflowToCase, to attach a block to a case.
You can attach a block to run as if it were a playbook using the ‘Attach Playbook to Alert’ action from the tools powerup, however this will not allow you to provide manual input to the block.