There is no expansion to convert Pandemic into Pandemic Legacy.
Firstly, the board is not the same anyway. It has a month track that doesn't exist on the non-Legacy board, as well as printed places for stickers that alter the board. Many other components are also different.
Secondly, even if there was an expansion/conversion kit, it would be nearly as expensive as the full Pandemic Legacy due to the large number of added components (sticker sheets, secret booklets, secret decks, secret boxes), and due to needing to replace some standard components with Legacy versions (some cards, the board, the occupation cards).
Thirdly, considering that you have to permanently alter your board and cards in order to play a Legacy game, you would just have to replace your copy of Pandemic after using it in a Legacy game anyway, resulting in no savings even if an expansion was offered.
No, the price of Pandemic Legacy is the only available way to play Pandemic Legacy.