Hi there, @byomtov.
At this time, there isn't an option to import Purchase Order (PO) and generate an estimate from PO.
In the meantime, you'll have to enter the estimate manually by going to the Customers menu and selecting Create Estimate in your QuickBooks Desktop (QBDT) for Mac.
Or you can look for a third-party application that can import PO and generate an estimate in QBDT for Mac.
You might want to check out this user's guide to learn more about entering transactions in QBDT for Mac: QuickBooks Desktop Mac User’s Guide.
Please know you can continue to reach me here with any additional questions. Thanks for coming to the Community, wishing you continued success.