Yes, you can add as many domains to OnINBOX Manager as you wish. Once OnINBOX is deployed to your primary G Suite or Office 365 domain, you don't have to go through the deployment process again for any secondary domains of yours. OnINBOX only has to be deployed on the primary domain and any secondary domains in the same tenancy will be able to make use of OnINBOX.
However, before you can make use of OnINBOX on a secondary domain, you will have to:
Let the Customer Success team or your dedicated Customer Success Engineer know what the domain is so that we can do the necessary to allow the domain to be used.
Add your domain to OnINBOX Manager