Thevinin and Norton equation simplifies BJT and MOSFET analysis leading to the hybrid pi models. Can we apply the same concept while analysing an AC-DC Converter with PFC and PSFB full bridge topology? For example, can I model a 100V rectifier having a thevinin resistance of Rth? If yes, how to calculate the Rth?
No I do not think you can.
The Hybrid Pi models are small signal models meaning the model is a linearization of the behavior of the transistor when biased in a certain state.
The Hybrid Pi models are useful for analyzing and designing circuits where the transistors are used as more or less linear amplifiers.
For circuits where the transistors are switching we cannot use these linearized models as there is no biasing applied.
The same is true for a rectifier diode. In power converters a rectifier diode will be subject to large signals so small signal models do not apply.
You could consider the behavior of a diode in one state like when it is conducting a certain current. It will then have series resistance which can be modeled as a series resistor in series with an (ideal) diode. I do not see how a Thevenin equivalent would help in that respect.