If you enter the stock symbol in Numbers, any information 'grabbed' is already contained in the same document. Nmbers has no means of communicating on its own with applications outside the open document or with other open documents, with two exceptions: You can enter a link in a cell that, when clicked will open a new email message in Mail, or a link that will open a specific webpage.
AppleScripts have been written that will collect stock data from a specified webpage and deliver that data to a Numbers the clipboard or to a Numbers document. These scripts need to be triggered by the user each time an update is required.
Two scripts related to this topic can be found in this (much earlier) discussion Re: Getting Stock Quotes Into iWork 3.1. The author, SGIII, is active, and will likely respond later today or early tomorrow.
Regards,
Barry