Custom character with user apperance?

You could use GetCharacterAppearanceAsync to get a variable containg the players model, loop through the accessories and attatchements and put them on the custom character.

Remember after you get the model you need to parent it somewhere such as the workspace.

local players = game:GetService("Players")
local success, character = pcall(function()   
return players:GetCharacterAppearanceAsync(player.UserId)
end)

if(success)then
        print("got player's character")
        print(character)
else
       print("could not get players character")
end

Aside from that, there is not a built in Roblox function to transfer the characters appearance directly onto the custom character.