Welcome to the forums.
The most obvious answer to your problem is to talk to your sales contact, they might be able to allow a larger number of users for your trial - how many additional users are you trying to create. I'm assuming that if the trial is successful you're going ahead with the Network Edition? If that's the case you're better off staying with that rather than converting, getting back to NE from OSS is slightly time consuming.
Anyway, all you do for a 'conversion' is to download and extract the OSS version then run the ./install.sh script - that's it.
