In Powershell, how do I convert an array of PSObjects to an array of Strings?

[string] $Name = $i.Name would store the stringified value of $i.Name in variable $Name, for instance - although it's surprising that that is needed, given that you're accessing a property named .Text on the Excel cell objects.

Generally, there are two basic ways to stringify (convert a value to a string) in PowerShell; assume the following two variable definitions in the examples below:

# Sample values to stringify.
$val = 1.2       # [double]
$coll = 42, 1.2  # array, containing [int] and [double]

• With PowerShell custom logic: culture-invariant, with additional logic for meaningfully stringifying collections, such as arrays:

  • Via a [string] cast or type constraint:

    [string] 1.2 # -> "1.2", irrespective of the current culture.
    
    # In combination with variables:
    $valAsString = [string] 1.2 # cast
    [string] $valAsString = 1.2 # type-constraint; auto-converts future 
                                # assignments to [string] too
    
    [string] $coll # -> "42 1.2", i.e. the space-concatenated list
                   #     of the (themselves stringified) elements.
    

  • Via an expandable (interpolating) string, i.e. inside "...":

    # Note: Only simple variable references as used here can *directly*
    #       be embedded in "..."; to embed *expressions or commands*,
    #       you must use $(...), the subexpression operator; e.g.:
    #         "$($val+1)" # -> "2.2"
    "$val"   # same as: [string] $val
    "$coll"  # same as: [string] $coll
    

• Via the .NET type at hand: potentially culture-sensitive:

  • Explicitly, via its .ToString() method:

    $val.ToString() # -> "1.2" in English cultures,
                    #    "1,2" in French, for instance
    
    $coll.ToString() # -> "System.Object[]", i.e. just the *type name*
    

  • Implicitly, via -f, the format operator:

    'val: {0}' -f $val # -> "val: 1.2" in English cultures,
                       #    "val: 1,2" in French, for instance
    

See this answer for more information.

Also note that PowerShell's flexible type conversions perform stringification on demand, e.g. when passing a non-string to a [string]-typed parameter.

As for what you tried:

Your code can be greatly simplified as follows; the source-code comments provide pointers, but explaining every optimization would be beyond the scope of this answer:

# Let PowerShell collect the loop output in an array for you,
$inputArray = 
  foreach ($i in 2..$rowMax) {
    # Construct and output a [pscustomobject] via a literal.
    # Note: If desired, you could apply [string] casts *here*; e.g.:
    #       Name = [string] $sheet.Cells.Item($i,1).Text      
    [pscustomobject] @{
      Name = $sheet.Cells.Item($i,1).Text
      Bot = $sheet.Cells.Item($i,2).Text
      PDD = $sheet.Cells.Item($i,3).Text
      SD = $sheet.Cells.Item($i,4).Text
      Dev = $sheet.Cells.Item($i,5).Text
    }
  }

# ...

foreach ($i in $InputArray) {
  if ($i.Name -eq $CurrentName) {
      # want to convert these name properties to strings
      [string] $Name = $i.Name
      [string] $Bot = $i.Bot
      [string] $PDD = $i.PDD
      [string] $Dev = $i.Dev
  }
  # ...
}