Using the Frequency Response Estimation tool in Simulink, how do I add an initial settling time to allow an external nonlinear system time to reach steady-state?

I have an S-function interface to an external nonlinear simulation package. I want to linearize the S-function block so I can design controllers. To simulate open-loop system, I first start the external program, then start the Simulink model which connects to and steps the external package. The nonlinear system takes about 60 seconds for initial transients to die out. I would like to linearize the Simulink model, including the S-function, using the Frequency Response Estimation (frest) tool. Waiting until t=60 to take a snapshot was straightforward, but I also want the tool to wait until t=60 before applying the sine sweep, and to ignore any system outputs for that time during the analysis. Does the tool have such capability, or do I need to perform the analysis offline? Is there a better way to go about this?

Thank you!

W.