@pbobadil
What is your exact use case here, you have different end dates for set of users and you wanted to update the same for them within Identity ? or do you have any post process to be done for the same.
If you just want to save the attribute at Identity level, you can set the attribute by getting identity object and save the object using below code
Note - You can place the csv file in server and read the file using basic Java code and then while iterating the identity and corresponding epoch time use the below code
Assuming identityName and epochMilliseconds are read already.
Identity iden=context.getObjectByName(Identity.class,identityName);
java.util.Date date1 = new java.util.Date(epochMilliseconds);
iden.setAttribute("contractEndDate",date1);
context.saveObject(iden);
context.commitTransaction();