Hi Dave,

I have discussed it with our developers, but your requirement is not clear enough.

You have to use an Advanced workflow if you want to handle custom steps. The third-party API should also return some value, right? I would recommend you to take a look at the following blog post: How to Create Custom Workflow Action.
Especially:

Arguments.StopProcessing = true;
using (WorkflowActionContext ctx = new WorkflowActionContext() { CheckStepPermissions = false })
{
	Arguments.Comment += string.Format(" | Document needs approval also from these manager(s): {0}", missingManagers.Join(", "));
	WorkflowManager.MoveToSpecificStep(Node, Arguments.OriginalStep, Arguments.Comment, WorkflowTransitionTypeEnum.Automatic);
}

with stopprocessing - you will stop the processing of request and using MoveToSpecificStep you can forward it where you need.

Best regards,
Martin Danko