USE [YourDB];   
OPEN MASTER KEY DECRYPTION BY PASSWORD = 'DecryptionPassword';   
BACKUP MASTER KEY TO FILE = 'c:\temp\exportedmasterkey.key'   
    ENCRYPTION BY PASSWORD = 'EncryptionPassword';  

param (
        [DbaInstanceParameter[]]$SqlInstance,
        [PSCredential]$SqlCredential,
        [PSCredential]$Credential,
        [string[]]$Database,
        [string[]]$ExcludeDatabase,
        [Alias("Password")]
        [Security.SecureString]$SecurePassword,
        [string]$Path,
       **[string]$ChildPath,
       [Alias("FileName")]**
        [parameter(ValueFromPipeline)]
        [Microsoft.SqlServer.Management.Smo.Database[]]$InputObject,
        [switch]$EnableException
    )
#
#
#
# pseudo
$ChildPath = $ChildPath or if null then "$fileinstance-$dbname-masterkey.key"` 
$filename = Join-DbaPath -SqlInstance $server -Path $Path -ChildPath $ChildPath