Yeah, we are create composer support for this legacy system.

1. save this json to your site root and as composer.json:

   {
     "name": "studioartcz/yoursite",
     "type": "project",
     "description": "Your site composer",
     "homepage": "https://www.studioart.cz",
     "private": true,
     "require": {
       "php": ">=5.6.0",
       "sasedev/composer-plugin-filecopier": "^1.1"
     },
     "extra": {
       "filescopier": [
         {
           "source": "vendor/studioartcz/sl_composer/plugins",
           "destination": "plugins",
           "debug": "true"
         }
       ]
     },
     "minimum-stability": "dev",
     "prefer-stable": true,      
     "config": {
       "bin-dir": "bin/",
       "discard-changes": true
     }
   }
   
   

2. create (edit) file .gitignore in project root contains:

   vendor/*
   bin
   config.php
   .idea
   data/*
   !data/cron.lock
   !data/installer
   pictures/*
   !pictures/pr
   !pictures/avatars
   !pictures/web
   upload/*
   
   # composer plugins
   plugins/extend/composer    
   

3. open command line at path where you have project's root

4. type: composer install studioartcz/sl_composer master-dev

5. yeah and you have composer your lucky human!