Just as "oratie" said, what you need to do are
1. setup the target/link library such that the tool can apply it.
2. constrain the design such that the tool know whether to apply it.
For more detail about (2), for example, if bottleneck of your design is power,
during the optimization, the tool will apply high-Vt cell to improve the leakage power.
While the bottleneck of your design is timing performance, the tool will some low-Vt cell to improve the timing until it met.