I am working on adding hash digest generating functionality to our code base. I wanted to use a String as a hash salt so that a pre-known key/passphrase could be prepended
If I'm understanding you correctly, it sounds like you've got it right. The psuedocode for the process looks something like:
string saltedValue = plainTextValue + saltString;
// or string saltedalue = saltString + plainTextValue;
Hash(saltedValue);
The Salt just adds another level of complexity for people trying to get at your information.