Yes, you should be able to do this with an Extension Attribute. When you created your EA, did you choose Text Field as the Input Type? Because that should allow you to go into every computer record and click Edit and then locate the EA and enter the text you need for that Extension Attribute and save it.
I would probably consider using a Pop Up Menu type instead, just to make sure the entry can be chosen from a pre-defined list and will always be consistent. Otherwise you run the risk of human error in entering the text.
Edit: In re-reading your original post, if you really just need all Macs regardless of their actual state to have that text, then a simple EA that outputs that text should suffice. Something super simple like this should work. Note that for an EA to output text the text has to be wrapped between XML result tags.
#!/bin/zsh
/bin/echo "<result>Data classification level: C2</result>"