In order to install PostNuke, you'll have to do it manually. Meaning that you'll have to download the latest PostNuke and follow the install directions that come with it. If it's similar to PHP Nuke, then there's prolly a .sql file that will create the tables you need to have that running. And how to do that is detailed in this How To. So far ppl haven't had troubles following it, so you should be good to go in that aspect. Plus the forums are awesome in helping you through any troubles you encounter.
The thing with Fantastico, like all the other scripts you can install with LunarPages is that it requires a fresh database. If you don't want to do the install manually, you'll have to purchase an additional database at the cost of $1/month.
It might seem daunting, but it isn't all that hard. Give it a try. Plus, make a backup of your database as it is now