You cannot. But don't worry. Let this round finish. Simultaneously, you can add all the missing assets to the inventory. These will not show in the current round.
When the current AR round is over, the system will assign a "Sell or donate" status to all the assets nobody was interested in. These assets will NOT appear in subsequent rounds.
Now if you create a second AR round, the divisees will see their old choices for the *other* assets ― those that at least one person declared interest in. If Tom selected Yes for the cupboard, then every divisee, Tom, Dick and Harry, will see the cupboard again. They will be able to change their bid for these items. The new assets that you entered will have no choice assigned yet, so they just need to complete.
Why don't we allow assets to be added to an ongoing round? It might become unfair very easily. We assume divisees are busy people, one of them might not realize he or she needs to amend their bid, or they might not have time to do it. The only fair way seems to be to create another round and give everyone a fair chance again.