You should be able to accomplish what you want, but it may take some experimentation, as the TTL circuit for the built-in flash (I'm assuming that's what you're referring to) might get confused. I'd set the Metz (as the "main" light) on full-power manual, and then meter that alone to establish the exposure (mainly the f-stop). Then, depending on what control you have on the built-in TTL flash, I'd dial that down at least a full stop, perhaps more depending on what you want, lighting-wise.
Remember, though, that the built-in flash will be a rather small, point light source, so it likely won't do well as a fill for large areas. It's probably OK for eye-socket and under-chin fill, however.