Since NeoLoad 5.4, you can use a custom Javascript when you would like to decode an extracted value with custom libraries.
In your variable extractor definition, select the option "Value is encoded as" and then "custom" from the drop down list. Then hit the "Edit decoder" button.
In the line var decodedValue = encodedValue + "string "; you can simply add your string value so it is appended to your extracted value.