You can create a new Menu to manage Forecast Updates:

Updating the Date Scheduled and Step from here gives you what you need.

How?

1. Create a Custom field for your Steps:

2. Create the new Menu using the button at the bottom of the Form view of the Stock Move model.

3. Create a Custom View and set it as the View Ref of the Window Action of that new Menu:

[Copy / Paste the XML from https://gist.github.com/ray-odoo/a21899d334dd899c4268286b999fc8b0]

4. Set the Context of the Window Action to:

{'search_default_future': 1, 'search_default_incoming': 1}

5. Create your own Custom Activities:

6. Create a User Defined default so all Stock Moves from your Purchase Orders start at Step 1.

Workflow:

Create a Blanket Order Purchase Agreement for your initial Products and Quantities (activate this feature in Settings if needed).

Your Inventory Forecast begins in Odoo when you place your Purchase Orders. From the Blanket Order, create a Purchase Order representing the accepted quantity and delivery date.

Our first PO will be for 50 of Widget1, 150 of Widget2 and 250 of Widget3 with Delivery Dates in July. Our second PO will be for 50 of each product with Delivery Dates in August.

Visiting our Menu, we can update the Step for the first 50 we ordered:

This screen works like a spreadsheet, click a cell to edit.

If you need to make updates to multiple PO's, Receipts or Products - you select them and use our "multi edit" feature:

Each time you update the Date Scheduled, the Forecasted Inventory Report will reflect the update and provide you with accurate information about when products can be expected as each of your Steps are completed.

To schedule Activities for those responsible for each Receipt, click the Transfer to open it and click Schedule Activity from the Chatter.

Extra Credit:

You can "pack" your products into Containers (activate Product Packagings) if you need to keep track at that level. 

HINT: This will need additional defaults in the Context dictionary for Product and Quantity to carry over to created Packagings.