In the Electrum wallet for pc, it is possible to modify a wallet from normal password to 2fA (two factor authentication ) without creating a new wallet?

No, it is not possible to add two-factor authentication to an existing wallet. these are two different wallet types.

the fee for the 2fA it's billed only when sending funds from the 2fA wallet to outside, and not when we receive, right?

The sender always pays the fee.