Task: Add converter for legacy `json` files

## 🥅 Objective

For legacy purposes, we should add a converter from the `json` inputfile format to the newer format and make it compatible, for the people that have been working with it.

## 📋 Todo List

Granular steps to finish the task

- Step 1
- Step 2

## 🧪 Tests

Which tests should be added to ensure the task works as intended

- Test 1
- Test 2
