My question is it passable to put a one more hart drive to de existing PC

Typical desktop or mid-tower format PCs support native installation of up to four hard drives. Whether it is possible in your PC depends on the PC and how many other drives are already installed. Additional drives can also be added by installing a drive controller adapter.

You can also add external drives via a USB connection

Yes. There are several variations.
Post the computer manufacturer's name, and model number. This way I can give you more than a generic answer.
Also post what size harddrive/s you have in mind, and how many.

(SATA adapter card = driver controller adapter )