When you say import your old site, do you mean importing just the content? Or importing the content and the look?
For the look, that would have to be rebuilt, a template would have to be created for whatever is running the site.
The content, there are usually tools that can help with that kind of thing but I don't know how good they are, I would imagine any tool would have to be tweaked by someone to get a good import, but having an automated process should be possible assuming the pages are mostly consistent with each other.
You would have to use the built in editor, most CMSs (content management system, Joomla is one of) the user generated content isn't stored in text files so can't be edited directly, the content is stored in a database. That said you should be able to embed anything that you create in the pages in the CMS.
Any reason for choosing Joomla? Wordpress is probably the most popular and pervasive CMS and you'd find the most support for it out there. The times I've used Joomla I haven't had great experiences, while Wordpress has almost always been easier and cleaner.
EDIT: Sorry should have refreshed before posting
