You can upload all of your past GL entries into your journal for categorization in Billy.
Head to Accounting > Journal > More Import
The file you will import should be a CSV file. I suggest Exporting the file first so you can have the template. This will tell you what headers Billy needs to import your data.
Once your data is cleaned up, with the headers Billy needs, and is saved as a CSV, import the file into the Journal. From there, you can choose the account and expense and income categories.
Below are a sample of some journal entries.