I have a spare domain name, that I want to use to try out your service. I'll load it up and run tests, and if the testing goes, well, then I'll switch my main domain over. The main domain needs to be "parked" on the same IP address as my test domain, when I decide to go live with my main site on Hostony.

People visiting the main domain should never see the test domain in their browser location (URL address) display.

Does your service support the addition of my main URL the way that I've described?

No problem with it.
You can set your main domain as "addon" domain later and point it to some folder of your spare domain, or you can set the main domain as "parked" or alias and have both pointed to the same folder or you can request me and I'll simply rename the domain in your account when you will be ready to transfer your main domain.

In every case your visitors will not see that URL changes somewhow or anything else.