Hi,
I have found a way to do it ! It needs a bit of time to do it.
1)Launch a race you want to custom then save
2)Go to your savegames (Documents/Assetto Corsa Compétizione/Savegames
and find the json called "XSE.json.
3)Search the section "carSet" and replace the lign under "info" by the lign present in the json of the liveries you want to race against.
example: i replace :
"carGuid": 100006,
"teamGuid": 100003,
"raceNumber": 6,
"raceNumberPadding": 0,
"auxLightKey": 0,
"auxLightColor": 164,
"skinTemplateKey": 104,
"skinColor1Id": 3,
"skinColor2Id": 13,
"skinColor3Id": 164,
"sponsorId": 0,
"skinMaterialType1": 3,
"skinMaterialType2": 1,
"skinMaterialType3": 2,
"rimColor1Id": 345,
"rimColor2Id": 13,
"rimMaterialType1": 1,
"rimMaterialType2": 2,
"teamName": "Racing Team 3",
"nationality": 14,
"displayName": "Racing Team 3",
"competitorName": "Racing Team 3",
"competitorNationality": 1,
"teamTemplateKey": 0,
"carModelType": 18,
"cupCategory": 0,
"licenseType": 0,
"useEnduranceKit": 0,
"customSkinName": "",
"bannerTemplateKey": 1
4) save and return to the game and load you've edited
5) Restart the race or the session.
This is now very convenient but for now it's the only way i found.
But i don't manage to make the game select custom skin directly.
If someone have a clue !