Yeah, I see, so you intend to run the internal RC oscillator for most of the time and switch to external crystal whenever you want to communicate with Sigfox?
I've never tried this; I've mostly played with ATMega and ATtiny uC's that don't support switching oscillators on the fly.
Have you considered using the external crystal, but downclocking during periods when speed isn't critical? Or, perhaps even more obviously - using the various power saving/sleep states of your microcontroller of choice? This would be the most obvious approach IMO.