There's a little bit more to it. The transformer impedance is, for the most part, caused by the primary winding resistance and the secondary winding resistance - both will affect the impedance feeding the remainder of the PSU.
In addition to this, you may want to add some limiting resistors. The best way I can think to illustrate this, is if you pull up the choke_input.psu file from the examples folder (on my machine it's?C:\Program Files (x86)\PSU Designer II\examples). This shows a source impedance per leg of 130 ohms.
If you want to have a 100 ohm limit resistor, simply change the source impedance to 230 ohms (130 + 100) and so on.
Should you need more info on how to figure out the 130 ohms impedance of the basic transformer in the first place, please shout and I can walk through some examples.
Regards,
Duncan
