Let me start by saying that I don’t recommend doing this at all.
Password Never Expires is bad security practice, but there are situations that might require it.
I had a similar request on how this could be done.
Setting it for multiple users:
#Connect of Office365
$O365Cred = Get-Credential
$O365Session = New-PSSession –ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $O365Cred -Authentication Basic -AllowRedirection
Connect-MsolService –Credential $O365Cred
#Get a List of user that belong to the second domain
$SDusers = Get-MsolUser -All -DomainName "yourseconddomain.com"
#Setting the password never to expire
ForEach($SDuser in $SDusers)
Set-MsolUser -UserPrincipalName $SDuser -PasswordNeverExpires $true
Setting it for a single user:
Get-MSOLUser -UserPrincipalName firstname.lastname@example.org | Select PasswordNeverExpires