I am searching for a method to add Joomla user to custom table (PrestaShop - same database) - duplicate user on registration. So user will have the ability to login to Joomla and 2nd script (PrestaShop) using same login information. Any ideas where I ought to start searching in Joomla files? I understand, that custom user wordpress plugin is a better method to achive user duplication, however I am searching for temporary hard-coded fix until I'll have the ability to write a wordpress plugin.

(Joomla 1.6)

See my response to your other question here: Writing Joomla bridge - User plugin

Another solutions will not allow you to save the passwords. If you're searching for saving the password too then you will have to consider

componentscom_usercontroller.php function register_save()

Note: I've given with regards to joomla 1.5. It could slightly vary if you work with joomla 1.6