If your signal ground is isolated from PSoC ground, then you can bring out the AGND to an external pin of PSoC and connect the signal with respect to this AGND.
If your signal ground is same as VSS of PSoC, then the signal can be connected to the input of PSoC using a capacitor and biasing the input of PSoC to AGND. Check out the below article that has more details on both these methods.
Measuring Negative Voltage Using PSoC1
The PSoC Hacker
Thank you very much for the fast response. My signal ground is not isolated from the PSoC Ground. So, I will use the C/R method of feeding the signal to the input of the PSoC. Thanks again for pointing me in the right direction.