adding huge number of owners into DL

I am currently supporting a huge account. more than 1000 users. hence, they have lot of DL lists. and some of the dl lists have huge number of owners.

i create a csv file with 2 fields. the DL_alias and member alias , then i run the script :

Import-Csv "C:\temp\Dl_bulk.csv" | foreach {set-DistributionGroup -Identity $_.dlname -managedby $_.mod}

problem is for set distributiongroup ,  If you want to add more than one owner to the distribution group, separate users by using a comma. If you don’t include the current owners in the command, the current owners will be replaced by the new list.

hence, my script will not work.

any tips?