It is possible to add a channel even if it's not listed in Sympl.
In fact, you are able to add any channel with a calendar - as long as it allows for exporting or importing iCal URLs.
Please follow the directions below to create an Unknown channel connection in Sympl:
Step 1: Create an Unknown Channel Connection
Navigate to the Channels module from the top menu bar.

In the Channels List view, click the + Add Channel button.

In the Channels Overlay, select the iCal tab and locate the Unknown option. Click Connect or on the + icon if there are existing connections.

Step 2: Export Sympl Calendar to the Channel
In the Exported Calendars section, click the Add icon.

Select the rental from the dropdown list and click Export.

In the popup, copy your Sympl iCal URL.

Paste this URL into the appropriate calendar import section in your external channel.
When the calendar export overlay page loads, you may select "Unknown" from the Channel drop-down list, then proceed as normal.
Copy the given calendar URL and proceed to import it using the channel of your choice.

The channel import process is the same as it is for any other iCal connection, but it's up to you to locate your calendar export URL in your channel of choice.
Step 3: Import the Channel’s Calendar into Sympl
Scroll down to the Imported Calendars section and click Add.

Complete the required information:

Calendar Name: Give your calendar a recognizable name.
Channel Name: Enter the name of the channel or select "Unknown."
Calendar URL: Paste the iCal URL provided by the channel.
Rental: Select the rental that should be updated with the imported events.
Once both import and export steps are completed, your availability in Sympl will sync with the external channel and vice versa.
You can repeat this process for any calendar-enabled platform that supports iCal syncing—even if it's not listed in Sympl .