Yes, you can. Your GFWL account is tied to your XBL gamertag, which means any points added to your gamertag are also added to your GFWL account, and vice versa. You can buy Microsoft Points either in card form, or, with a credit card, through the GFWL or XBL client as well. Again, points purchased in this way are shared between your GFWL account and XBL account.
On that note, whoever invented GFWL needs to burn and die in an extremely painful manner.