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.
$a = gci -Path C:\inetpub\iislogs3.com\wwwlogs\test -Filter *.log
foreach($file in $a)
Write-Host "Processing - $file"
$a = gc $file.FullName
Add-Content -Path c:\temp\loadtest.log -Value $a