Yes, you can accept your invite using your existing ePACT account. Just click the orange Complete Request button in your email invite and log in using the email address tied to the ePACT account that you'd like to use.
The email address to which the invite was sent will be auto-filled. Simply delete it and replace it with the address you used to create your account.
Please Note: If you do not have an existing account, you will need to create an account by clicking on Sign Up, and then follow these instructions to change your email address.