Hi Tammy,
Yes you can take your old joomla website, install wordpress on it and use it as a development website.
However we can’t really give you generic support on how to use wordpress or joomla here. A quick search on google will pull thousands of website talking about how to move a joomla website to a wordpress website.
In this forum We can give you support on how to use GeoDirectory only and you are supposed to know at least the basics of wordpress to be able to do that.
One altrernative to a test website that I can suggest as solution to your problem, is to install a maintenance plugin that will present a coming soon page to your visitor, while you change theme and modify your website.
Thanks