Let's take an example

You create a sale Order for "Consultancy"

You record a bill for a "Risk Analysis" provided by a supplier

You wan to invoice this bill to your customer

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1) Create a sale order with the Consultancy Product

2) Define an analytic account 

Note: If you configured your consultancy product as a  service + service tracking ( example: create a task in sales order's project) Odoo will create an analytic account for you.

2) Create a product "Risk analysis"  with "can be expensed" = True

3) Create the Bill with the product "Risk Analysis"  + Select the analytic Account  defined on your sale order

4) The product from your bill is added on a new SO line on the original SO. 

5) Create the invoice from the sale order