Can I convert a flat-bar road bike to drop bars later?

Technically yes, but it is expensive and often not cost-effective. You would need to replace the handlebar, shifters, brake levers, stem, and potentially the front derailleur and brakes — essentially the entire cockpit and control system. The cost of parts and labor often approaches the price of a new drop-bar bike. If you think you might want drop bars in the future, buy a drop-bar bike now.