Is it possible to enforce a break in the city to city route with a gap?

I am wanting to cover a bit of South America, but oneworld has no presence. So trying to fly into SCL or EZE and out of UIO (or vice versa) and considering filling the bit between SCL/EZE and UIO outside of the OW RTW ticket, but the online planner is not letting me. My intended RTW is proving difficult to put together.

Thanks.