I used this script to read a few months worth of my IIS logs to create a single log file.  I used the single file for load testing with Web Application Stress tool. 

$a = gci -Path C:inetpubiislogs3.comwwwlogstest -Filter *.log

foreach($file in $a)
{
             Write-Host “Processing – $file”
             $a = gc $file.FullName
             Add-Content -Path c:temploadtest.log -Value $a

}

Cheers,

Steve