If you've already set up a Google Apps for Work account outside of POP that you'd like to use for your domain, you can link it to your domain name in your POP account.
Note: DNS information is managed by unlocking Full Domain Control. You can unlock your domain by upgrading your Simple DNS plan to Full Domain Control. To learn more about Full Domain Control, click here.
You'll need to verify for Google that you're the owner of the domain. Google will provide a TXT record for verification that you need to add to your POP account. To add this in your POP account click Domain Settings on Domain Name.
Click on the Advanced DNS tab to create the verification record. Under Custom DNS records, click Add New Record to get started
Create a TXT record using your Google Verification TXT value found in your Google Apps for Work account, in the format shown below:
You'll need to create MX records as well for Google Apps, your final setup for email records should look like this:
That's it! Once you've set up these records you should be able to test your send and receive from your existing account!