They are already the same so long as you are using one email address to collect points (both in-store and online). See below for more information.
I've made purchases in-store but never provided my email address: You'll need to create an account with the email address you want your account tied to and use this email address going forward for all your purchases. You can create this account in-store simply by providing your email address during your next purchase, or you can create it online by clicking Become a Naturalist above. If you create your account in-store first, you'll still have to create it again online so you can set a password. It will be the same account as long provided you use the same email address!
I've made purchases in-store with an email address, but never online: No worries, your account still exists online! You'll just need to create a password for it. Click on Sign Up, enter the requested information but be sure to use the same email address you've previously used in-store (or the one you intend to use in-store going forward!)