I was wondering if there is any advice on how to implement a variable pole (low pass filter) in an average model? I was thinking using a select line based on thresholds which would allow to switch between different poles, but wanted to ask if there is an alternate method (and more efficient!) to do this.

The reason for this request is that I am trying to model a variable frequency system where the pole is moving based on the switching frequency.

A low pass filter has the transfer function G(s) = 1/(Ts+1). Typically you would use a Transfer Function block to model this in PLECS. To be able to control the time constant T with an input signal, you need to implement the transfer function with discrete Sum, Product and Integrator blocks.