In a word - No.
I'm not familiar with the I2S interface.
If you want a pure tone, you will need to generate a sine wave. A square wave contains many harmonics caused by the voltage steps. If you just want a siren sound, a square wave maybe good enough and the easiest to setup.
In the sine wave generation, I'm talking about digital signal processing. Unless you use the processor, you will need to add hardware to create a sine wave. I would recommend you look at the AD9834 chip. Using the processor, you need a constant update rate (i.e. send a sample each time an interrupt occurs) to send sine wave samples to a D/A converter. Either the built-in D/A (if it has enough resolution) or some external D/A like a MCP4802.