I have lots of domain name and use G Suite for a single domain. I needed to create a mail account for one of another domain names, and tried to find a way to combine with the existing G Suite domain. Following procedure enables you to combine your G Suite with multiple domain names.

STEP 1. Connect to Google Admin at https://admin.google.com/ac/home

STEP 2. Click "Domains" → Click "Manage domains" → Click  on top of the page

Then you will see following diagram, and you can put your domain name that you want to combine with the existing G Suite domain name.

STEP 3. Verify your domain name

Then you will see below page. In my case, I just selected "Other" for domain name provider, and copied the TXT record below to the DNS configuration.

In my case, I use dnszi.com, so simply did as following:

It may usually takes 2 hours, but may takes 72 hours in some cases - that is usually based on the TTL(Time To Live) value defined in the DNS configuration.

If every thing is okay, you will see message like below:

STEP 4. Setup MX record in the DNS configuration based on the guide from G Suite once verification is done on STEP 3

You can simply do it by click "Set up google MX records"

Below is the list of MX Server address I got

MX Server addressPriority

And below is what I have done at dnszi.com - you may able to do that like me based on your environment

If everything is okay, now you can receive an email in a single mail box with multiple domains.

You can also add mail alias if you want it. For example, you add foo@your_domain.com to your_original_email@your_domain.com. It can be done at "Users" on Google Admin