Yes, you can. A private key is basically a 256 bits number from which you derive the public key then you derive the address.
Nearly every 256-bit number is a valid ECDSA private key. Specifically, any 256-bit number from 0x1 to 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4140 is a valid private key.
https://en.bitcoin.it/wiki/Private_keyThe easiest way to derive a private key from a passphrase is to hash it using sha256.
There are many online tools that let you generate a bitcoin address from a passphrase like keybase.io
I don't advice you to use an online tool, though. Find an open source code and run it offline.
Here is an easy tutorial on
How to generate your very own Bitcoin private key and on [url=https://coindoo.com/how-to-create-a-bitcoin-wallet-address-from-a-private-key/]How to Create a Bitcoin Wallet Address from a Private Key[/url]