New products should be added in your store first. Once they've been created, you can run a variant sync in Inventory Planner to pull in the new information.

Click on the Sync button on the bottom left-hand side of your screen, then on Variants sync.

If you have products that aren't sold on your store but still need to be replenished (e.g. assembly components), you can create them in Inventory Planner as IP variants.

Learn more about IP variants here.

If you need to add an item to a purchase order that isn't in your store yet, you can do that with the "Add non-existing product" button.