Few weeks ago I needed to tabify (change spaces to tabs) in a C# files in a solution. I tested some plug-ins to Visual Studio, but none of them did what I wanted. I left the idea, as it was not that important to have consistent tabs/spaces. But a day or two ago I had some time and yen for to create simple PowerShell script to fix that for me. It simply replaces x spaces taken from input parameter with tab(s). Text goes to stdin and result to stdout.
param([int]$spaces)
$pattern = '';
for ($i = 0; $i -lt $spaces; $i++) {
$pattern = $pattern + ' ';
}
$pattern = '^((' + $pattern + ')*)' + $pattern;
$replace = '$1' + "`t";
$input | foreach {
$text = $_;
do {
$prev = $text;
$text = $text -replace $pattern, $replace;
} while ($prev -ne $text)
Write-Output $text;
}
The script, as you can see, is not interpreting the code in any way, hence some spaces i.e. in comments at the beginning might become tabs even if that’s actually wrong. But hey, you can take Roslyn and plug it in, as an exercise. 
To, for instance, tabify all .cs files in some path, you can call this script (saved as tabify.ps1) using something like:
Get-ChildItem -Recurse ".\src" | Where-Object {$_.Extension -eq ".cs"} | foreach { Get-Content $_.FullName | .\tabify.ps1 -Spaces 2 | Set-Content -Encoding UTF8 ($_.FullName + ".tabs") }