iCloud functions as a CalDAV server that hosts your calendars and syncs them with client apps. BusyCal and the built-in Calendar app on OS X and iOS are CalDAV clients that sync with iCloud.
This means any data you create on an iCloud calendar using the built-in Calendar app on OS X or iOS will sync to BusyCal through iCloud, and vice versa. As long as you are hosting all of your calendars on iCloud and have all client apps configured to sync with the same iCloud account, all of your calendar data will remain in sync.
All calendars you wish to sync between your Macs, iOS devices, and other users, must be hosted on iCloud. You cannot sync calendars from other sources (On My Mac, Google, Exchange, etc.) to iCloud.
If you have any existing calendars hosted elsewhere that you want to host on iCloud, you can move them to iCloud or merge them with an existing calendar in iCloud, as follows:
For this example, let's say you have a local calendar called "Work" that you want to host on iCloud. Here's how you can move it to iCloud:
Make sure you are syncing with iCloud and your iCloud Account appears in the left sidebar.
Control-click on the calendar that you want to move to iCloud and choose Move to iCloud.
iCloud requires you to store Events and To Dos on separate calendars. If the Work calendar contains both Events and To Dos, it will be transformed into two calendars on iCloud — one for Events and one for To Dos. The To Do calendar will be indicated with a checkmark next to its name.
For this example, let's say you have a local calendar called "Work" that you want to merge with an existing calendar on iCloud called "Fred". Here's how to do it:
Export the Work calendar by selecting it in the left sidebar and choosing File > Export, and save it to your desktop. Then delete the Work calendar from BusyCal by control-clicking on it and choosing Delete.
Select File > Import, and select the file you exported in step 1. On the bottom of the import dialog, select the option to Import events into Existing Calendar, and choose the Fred calendar (which is hosted on iCloud). This will add the events from the Work calendar to the Fred calendar on iCloud.
Note: iCloud stores Events and To Dos on separate calendars. Therefore, if you attempt to import a calendar that contains both Events and To Dos into a calendar on iCloud that only supports Events, the To Dos will be rejected. To import the To Dos, you must repeat this step and import into an existing calendar on iCloud that supports To Dos.
Tip: After merging the calendars, you can remove duplicate events by selecting Help > Find Duplicates.