Yes, it's possible. PSoC5 has analog modulators to demodulate incoming AC signal into I and Q-components, and ADCs to sample them. It would save time if you describe details of your project first, things like signal amplitude, 100kHz reference source availability (both I and Q quadrature), required bandwidth, bit resolution, etc. - the more the better.
The essential part of the full-quadrant lock-in detector is quadrature generator. You can find a demo project for PSoC quad gen operating at 100 kHz in this thread
The generator synthesizes quadrature-shifted copy of the external 100 kHz clock. Together with input clock, they make I/Q pair for driving PSoC analog modulators or an external Mux.
Fig 1. Yellow - input clock, Cyan - quadrature output