sticker-convert supports downloading viber stickers from share link of sticker pack.
sticker-convert can also convert images to png with 490x490 for uploading to viber manually. It should be noted that Viber is able to resize images for you, so it may not be necessary to use sticker-convert for creating Viber sticker pack.
For more info, please refer to https://help.viber.com/hc/en-us/articles/9204828903837-Use-and-create-stickers-on-Rakuten-Viber