can i download and add extension for "share with" ?

[supergaia]

hi
i'm reading https://support.apple.com/kb/PH25749?viewlocale=en_US&locale=en_US

macOS Sierra: Share on Twitter, Facebook, LinkedIn, and more

can i download and add extension for share with?
i mean i would like to have 500px (it's a photo hosting service like fickr) and VK (it's like facebook)

i'm new to mac , i came from windows , are they called extensions? where can i add and download them?

thanks