Add additional values to an existing property of a PSCustomObject

I have started to use PSCustomObjects. I am reading the information from a CSV with lines of data and a header row. I would like the ability to add addition values to the SAN property after the fact. There is no problem with more than one SAN property if it is read from the CSV. If I try to supplement the existing values with additional values no changes are made. How can add additional values using PowerShell and PSCustomObjects?

#initial values from reading the CSV file.
$PRODCO = [PSCustomObject] @{

                       SAN            = $PROD.San
                       SANENV         = $PROD.SANEnv

                       }

SAN            : ftpgoals.company.com
SANENV         : PROD
#values after reading from cell with two SANs
$PRODCO = [PSCustomObject] @{

                       SAN            = $PROD.San
                       SANENV         = $PROD.SANEnv

                       }

SAN            : ftpgoals.company.com, www.prod.ftpgoals.company.com
SANENV         : PROD