you can't force an app to become transparent, because that's up to the developer to enable.
Also, being live and transparent are independent of each other. (MSN Weather has a live tile but is opaque. Maps is transparent yet "dead")
however you can replace it with a transparent tile using one of those transparent tile apps, which ironically aren't live.
Examples include 8.1 HD Tiles and Start Perfect, but neither have that app. I don't know which one does...