How to append strings to other strings in a data set?

You could use something like this:

Example 1

$t = "test"
$t = $t + ".com"

Example 2

$test = @("test1","test2")

$test | ForEach-Object {
$t = $_ + ".com"
Write-Host $t}

With your added code I did this. I don't have a database to test it on, so I made the data set manually, so you might just have to change the $DataSet[0] in my code to $DataSet.Tables[0].

$DataSet[0] | ConvertTO-Csv -Delimiter ',' -NotypeInformation  | Foreach-Object{$T=$_
IF($T -match "(Cherry\d\d)"){$T = $T -replace "(Cherry\d\d)(.+)",'$1.com$2'};$T } |  out-file  $outfile -Encoding "unicode"

There are many ways. Here are a few:

# Using string concatenation
'Test1','Test2','Test3' | Foreach-Object{ $_ + '.com' }

# Using string expansion
'Test1','Test2','Test3' | Foreach-Object{ "$_.com" }

# Using string format
'Test1','Test2','Test3' | Foreach-Object{ "{0}{1}" -f $_,'.com' }