Can Excel VSTO add-ins call Excel web add-ins?

Excel VSTO add-ins can not call to Excel web add-ins. However, there is a way to run the Excel COM add-ins in a compatible mode on Windows, while running the web add-ins in other platforms (Mac and Online). Please check out this document: Make your Office Add-in compatible with an existing COM add-in. For custom functions, similar mode also works for functions defined in XLL, please check out this document: Extend custom functions with XLL user-defined functions