Here's a how-to, but you should know some differences between internal categories and website categories. See at the bottom.

You can export then import, but this is not so easy if you have sub categories:

1. Go to Website > eCommerce > eCommerce Categories
2. Select all categories, a little Action button will appear above the list
3. Choose the Export action
4. the new window show available fields on the left and exported on the right ; remive evrything from the right
5. add fields you want to export ; if you have sub-categories, you will need to try a few times to got the exact fields you need
6. you will got a spreadsheet with field names at the first line

The first column should be the categories ID, remove it. Now you can import this list to the Internal Categories. But you will need to either change the field name in the spreadsheet first line before importing or to directly choose the fields in the dropdown menu in the import window.

You may also do this with product export then import:

1. Go to the product list, select all of them and choose the Export action
2. Export the website categories fields AND the Internal Categories fields
3. Open the resulting spreadsheet ; the first column of the spreadsheet will contain the product ID. DO NOT REMOVE IT.
4. Change the name of the Website Categories fields with the Internal Categories fields
5. Keep only the first column with product ID and the columns containing the now populated Internal Categories
6. import back ; Odoo will create the Internal Categories

Internal Categories versus Website Categories

• a product can belongs to several website categories, but only one internal category
• website categories are used only for website browsing and are unrelated to internal categories
• internal categories are used with pricelists, accounting, budgeting, etc. with a lot of classical companies operations.
• internal categories can be used to set conditional default values (you must enter debug mode first)
• internal categories don't need to be as detailed as the website categories, sometimes nearly all products belongs to the same internal category

Hope this help