Technically the people in the wrong (but not illegal; it's against the rules, not the law) are anybody who creates and/or shares a tweak app without licence protection. It doesn't matter which forum you release it on. Of course being the official forum, this is one place such a rule will always be enforced. Finding such an app and using it is no problem, it's the app makers/distributors we frown upon.
To the topic; just edit your cars as usual. I've not seen an app that modifies the dedicated server, so you'll have to use a full client for serving. Of course anyone joining without an identical set of modifications will be booted upon joining. So you have to share preset files.