Tokens when setting up activity scheduled reminder
Yes do add additional values to the template you need to create a tokens on your own by creating new extension or by adding token hook in the existing extension for example for creating tokens please refer to the below link 

https://wiki.civicrm.org/confluence/display/CRMDOC/Customized+%28and+Custom%29+Tokens

https://book.civicrm.org/user/current/common-workflows/tokens-and-mail-merge/

If you haven't created New extension please refer to the below link 

https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension