Yes, you can.
To enable your encrypted system not to ask for a password at boot-time, you first need to move the encryption key from your system volume to external media (e.g. USB stick), then change the password for the volume to an empty one.
After that, your computer will boot with the external media connected, but will not boot if the media is not connected.

For detailed instructions on how to move an encryption key to external storage, please see