Not referring to the Internal Oscillator at all.
The PSoC 3, 5 & 5LP on their datasheet have XTAL pins.
The PSoC 4 only has and EXTCLK pin meaning that it can take an external clock, but you can't connect a crystal directly to it, this is done by design.
Is there a way to use, say, analog input and output pins to do it, or some obscure configuration? maybe, but that's not intended so it can't be regarded as a solution.
So he can add a darlington based crystal clock and feed the external generated clock to the chip for 5 cents more on parts than hooking it directly, or forget about it.
By driving it I mean making the XTAL resonate and give the needed clock frequency. The PSoC 4 is not designed to do that, but it does however take an external clock to override the internal RC clock.