How can I modify the existing report template?
These two fields would be generally useful to many CiviCRM users, so I'd recommend adding them to the core report just before https://github.com/civicrm/civicrm-core/blob/master/CRM/Report/Form/Contact/Detail.php#L85. 

Here are some instructions on using GitHub to contribute to core CiviCRM: https://wiki.civicrm.org/confluence/display/CRMDOC/Contributing+to+CiviCRM+using+GitHub

You can get the names of the fields at https://github.com/civicrm/civicrm-core/blob/master/xml/schema/Contact/Contact.xml#L162 and https://github.com/civicrm/civicrm-core/blob/master/xml/schema/Contact/Contact.xml#L359. 

Just adding the field definitions should be sufficient to add them as data columns...a bit more work is required to make them available as order by fields. Once you have tested the changes, please submit a PR. You can ask me for help reviewing the contribution by mentioning @JoeMurray in a comment on the PR. Good luck!