Limit access to update properties for a user group in a workflow

Summary:

I would like to set up my permissions for my B&P user group so that B&P are the second in a 3 stage workflow where their only access is to insert the property into an alternate hierarchy.

Content (please ensure you mask any confidential information):

What I would like to do is.

1. An initial request to add a new node type is submitted to workflow. This request inevitably includes updated property values in it.
2. The request goes to B&P if the value needs to be inserted into an alternate hierarchy. B&P should only have the ability to insert the node. I don't want them to be able to change any of the properties on the request.