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