Hello
I will outline the solution for you and if you find it as per your need I can start on your project:
1. You have few Open Cart Tables in the database say A, B, C
2. You have some Joomla core tables in the database say J, K , L
3. You need to select few or all fields from A,B,C and you need to select few or all fields from J,K,L
4. This new selected fields will be available to you on joomla back end screen
5. If you want this new fields can be viewed, edited or deleted.
I will deliver this solution as a joomla back end component which will be user friendly and easy to manage. The coding will be done using PHP, MYSQL, CSS and some Jquery if required. If you show interest, we can discuss this further.
Thanks
Bimlesh