The pss fundamental (beat frequency) can be setup as a design variable and then swept in a paramteric analysis quite easily achieve a swept frequency pss.
But in your case I would ask you what simulation you are actually trying to do ?
If you really want large signal LO & non-small-signal RF and have a fixed IF frequency, then the IF will be non-small tone so you would try to choose the pss beat frequency as the IF frequency (assuming its quite large, otherwise the simulation will be very ineffiecient).
If having done this, then you could now quite simply parametrically sweep LO and RF frequency in IF Frequency steps to cover your LO/RF frequency range of interest, so that the pss beat frequency does not change between runs.

Note that you can also sweep one of the frequencies (say the LO or the RF) using the sweep on the PSS form - and the other using the parametric analysis form.

For the beat frequency, put some nominal value (it doesn't really matter - just do auto-calculate, perhaps).

Then in the Options form, down the bottom, there is a field labeled "Additional Parameters". Enter "autofund=yes" in that field.

You need to be using at least IC5141 USR2 or later for the additional parameters field to be present. And you need to be using spectre from MMSIM60 or later for pss to support autofund. This tells pss to calculate the pss fundamental from the sources in the circuit, rather than using a specified fundamental frequency. The one place this won't work is if you have a divider in the circuit - if you do, then add a dummy source in the circuit at the divided down frequency.

Regards,

Andrew.