You can add skins via the config.
I removed the skins that used to be included when the workshop TOS changed.
there is "static_skins" under each set type. These take a shortname/skin id.
For example, if you wanted the Miners buff to always drop with a specific pickaxe skin:
"static_skins": {
"pickaxe": 3563165226
},
The other option is to have a pool of skins that assign to all items randomly.
"List of skins for item types. If more than 1 skin is added, a random one will be selected when the item is created.": {
"roadsign.kilt": [
3571624896,
3571002689
],
"coffeecan.helmet": [
2597330272,
1293654354
],
...etc
}