Zend Optimizer is free, so I don't know why they don't add support for it.
But regardless, there are tools/services to decode something encoded with Zend Guard. I've heard Dezend is one of the best among them, but I've never tried it. Most likely, you'll either get a completely working script, the basic form of the script which you have to complete, or an unusable version of it.
If you know php, it's probably better to try to rewrite the script. If you don't, try using a decryption tool(don't pay for anything). And if that doesn't work, explain what the script does here and perhaps someone will make it.