If you have measured the R1+R2 of each circuit and measured the continuity of the supply tails and earth (generally zero on our meters) and know the Ze from the permanent supply then you have a calculation for Zs for each circuit.
There is no requirement to actually measure a live Zs reading, though it is often done as a confirmation and to provide a real world value and in some cases is considered an essential measurement, but it is only one of the methods specified.
The measurement by calculation is the worst case scenario.
It would certainly not be a limitation as a new installation cannot have any limitations.
If you want to connect to the permanent supply then run interconnecting cables suitable for the load from the load side of the off main switch on the permanent supply to the load side of the main switch on the off peak supply. Then switch on the permanent supply main switch. This should be done with all due care and prevention of exposure to risk. This will also give a slightly different (very small) because of the different cable run.