I want to set when updating the workflow list without using Explorer.
You can use the update data activity after the list creation to set the expiry date.
In case you want to have a computed label for your List,
1.you can store it in a instance variable first
2. set the label using instance variable.
3. set expiry date using a update data activity as per the following.
Yes You can set this up. Just create a workflow that read list table and select list without expiration date and update them with a data update activity.