Official comment
The ONTRApages Free account is limited to the ONTRAPORT domains we provide. The ONTRApages Premium account ($15 per month or $99 per year) allows you to use your own domain, but you are right, that domain is only used for the ONTRApages landing pages. If you have existing content on that domain it will "disappear" when you change the name servers to point to the ONTRAPORT servers.
If your existing content is in a WordPress site you can use the ONTRApages plugin to host ONTRApages on that site. The plugin is the only way we approve to have your own content and an ONTRApage on the same site.
There is a trick we no longer recommend, using a wildcard A record to create subdomains on the fly on your existing domain. We used to explain how to do this, but too many people skipped a step, or mis-configured their domain, and lost their content. It is an option is you have a DNS ninja handy to walk you through the process (and if they have never heard of a wildcard A record, they are not the ninja you are looking for).