You can copy calendar items from one Outlook account to another using the Mac Calendar app. 

1. Select the calendar you wish to copy events from (it’s recommended that you have the other calendar’s visibility disabled), and click to select an event (shift+click to select multiple events).
   
   
2. Then right-click or control+click to pull up the pop-up menu. Choose Copy. You can also drag the calendar event over to the other calendar manually.
   
   
3. Hide the source calendar, and make the new calendar visible. Click on the day to use as the first day that will have events pasted from the other calendar, right-click or control-click, and then choose Paste. It should paste all the items you copied into the same pattern that they appeared in the source calendar (for example, if you selected items from the first Monday in the month, and then additionally, items from 2 days later, then click on the first Monday in the month on the new calendar and paste, it will paste all items on the correct days and time periods.

Note: You can select single calendar events or multiple batch events to copy over to the new calendar. However, you cannot drag your entire calendar over to another in the Mac Calendar app. In this instance we recommend you use the import/export options.