Can we install/side-load outlook custom add-in for Mac programatically without manually installing
Is there a way to install outlook custom add-ins in Mac from terminal without requiring manually installation.

Yes.  From Microsoft Documentation:
Sideload Office Add-ins on iPad and Mac for testing
For side loading the add-on in Office for Mac, you need to save/copy the manifest file to the wef folder. If the folder doesn't exist, you must create it.

Word: /Users/&lt;username&gt;/Library/Containers/com.microsoft.Word/Data/Documents/wef

Excel: /Users/&lt;username&gt;/Library/Containers/com.microsoft.Excel/Data/Documents/wef

PowerPoint: /Users/&lt;username&gt;/Library/Containers/com.microsoft.Powerpoint/Data/Documents/wef