Add-Content for each line in a CSV using PowerShell

Since you are using Import-Csv to read your file, which I assume is a proper CSV, you can use Export-Csv to output custom objects.

# Open file
$file = Import-Csv 'filepath'

# Loop through each line of CSV
# store results in $resultTable
$resultTable = foreach ($line in $file)
{       
    # Assign file path, and regex just the extension
    $path = $line.path
    $extension = $path -replace '.*\\Extensions\\'

    # Open chrome webstore for specific extension
    $result = Invoke-WebRequest -Uri "https://chrome.google.com/webstore/detail/$extension" -Method Get
    
    # Grab title of extension and place in CSV
    $title = $result.ParsedHtml.title
    
    # Create and output custom object with path,extension,title properties
    [pscustomobject]@{
        Path = $path
        Extension = $extension
        Title = $title
    }
}
# Export to CSV overwriting 'filepath'
$resultTable | Export-Csv 'filepath' -NoType

Export-Csv converts an input object into a CSV. Each property of the object becomes a column and each property value is output under those respective columns. Each object becomes its own row in the case of an array of objects.