The Self-Storage Development model you reference wasn’t specifically designed to handle value-add scenarios, but it is possible.
Enter the purchase price of the existing project under Land Costs. Include the construction budget and schedule for the value add component under ‘Development Period Cash Flows’, and then turn on ‘Detailed Lease-Up Method’ (cell H71 of the Underwriting tab) to manually model the lease-up of the expansion units.
Again, not a perfect match but certainly possible.