Using stock skins and own skins at once is currently not supported. Please add your own “stock skins”.
If you enable signups on the event, files and entries can be created by the participants themself: https://wiki.apx.chmr.eu/doku.php?id=event_signup
You can also add the entry by yourself:
The field “base class” is only needed if a mod has multiple specifications, such as the GT4 and Cup ones in the A110 mod.
Now you have to upload the files for the entry (liveries most likely). You can add them in the entry files section. Select the newly created car (e. g. the #123 from the last step) and upload files.
You have to repeat this step per file needed. For larger amounts of files, there is an import available: https://wiki.apx.chmr.eu/doku.php?id=import
In the event, select your newly created entry in the entries section.
Go to your desired server, select the event and update the server. Everything else will be taken care by APX.