Add User to Exchange In-placeHold/eDiscovery (In-Situ) with powershell - powershell

Question

I'm currently encountering some problems with powershell, while creating scripts.
I want to add a User to our In-place-Hold / eDiscovery. I more or less found a workaround to adding a single User to the list, but I'm not sure if it works without losing data of the already existing In-Place-Hold users.
The goal is to add newly created users to an existing In-Place-Hold.
However, maybe its easier to understand with my code:
#All users that are already in the in-situ
$check = Get-MailboxSearch “In situ autotest"
foreach ($User in $ADUsers)
{
$Username = $User.username
$Firstname = $User.firstname
$Lastname = $User.lastname
if (Get-Mailbox -Identity $Username)
{
#If user does exist, output a warning message
Write-Warning "Benutzername $Username already existing on the Exchange Server."
}
else
{
Enable-Mailbox -Identity "$Username" -DomainController 'DC.domain.com'
}
#search for the user
$Add = Get-Mailbox -Identity "$Username"
$check.sources.add("$Add")
}
#write all users back into insitu mailbox
Set-Mailboxsearch -Name "In situ autotest" -Identity "In situ autotest" -SourceMailboxes $check.sources
Does someone know if there is an easier way to add a single user to the sourcemailboxes of Set-Mailboxsearch, without having to aplly everyone again?
Thank you guys very much in advance!