There is no passive voice version for this sentence. A passive sentence focuses on the object of the verb, meaning there should be an object in your active sentence, so that you can make it the focus of the new sentence. In other words, passive voice is only possible for TRANSITIVE verbs (verbs that take a DIRECT object). The verb "work" in the sense used in your sentence, is an INTRANSITIVE verb. You don't have a direct object upon which you can focus your new sentence. If you want to focus your sentence on the company, you can say: "The company I work for", but that wouldn't be a passive sentence. Look at this example: I was walking in the garden. Can you turn it into passive? ;)

The company that i'm worked for

The company, I am working for...

The company was worked for by me ^^ A sentence of this type should exist in Tagalog and other Austronesian languages...