The trucksim data is inadequate to generate a useful psd because the sampling frequency is much lower than required.  Have you done a Modal analysis of your structure?  What is the frequency of the first mode?

Instead of trucksim data, use a published psd table for truck transport on a rough road. I expect you could find that on the internet.

The easy way to convert an excel worksheet to an ascii file is to Save As a Comma Separated Values (.csv) file type.