When you read multi-valued information from WMI or any other source, for example, network adapter IP addresses, this information is returned as a multi line string:
PS> Get-WmiObject -Class Win32_NetworkAdapterConfiguration -Filter 'IPEnabled=true' | Select-Object -ExpandProperty IPAddress 78.64.118.150 fe80::ad62:ac4d:4dea:936d
If you want to turn this into a list, use the operator -join:
PS> (Get-WmiObject -Class Win32_NetworkAdapterConfiguration -Filter 'IPEnabled=true' | Select-Object -ExpandProperty IPAddress) -join ', ' 78.64.118.150, fe80::ad62:ac4d:4dea:936d
It expects the multi line (array) data on its left side and the delimiter you want to use to separate the values on its right side.