Is it possible to transfer your Pokemon Go account to Trainer Club?

You can now link PTC, Google, and Facebook accounts: From the FAQ:

If you would like to use a different login method, you can link another login provider to your account. Learn more.

In Map View, tap the Main Menu At the top right, tap Settings Under the Account section, select the “Not Linked” checkbox next to the login provider you would like to connect. Follow the prompts to link your account. Once your account is linked, you can use that login provider to sign into your Pokémon GO account.

From the Pokemon Go FAQ

"I would like to switch my login method"

At this time, you cannot transfer your account’s login method from a Google Account to Pokémon Trainer Club or vice versa.