No, no way to add more. What are you using them for?
One possible workaround is to become a supporter and then use the Manual Entry of Spending Changes at the bottom of the Spending Models tab. Use a spreadsheet to calculate your spending each year net of the variou off-chart items that you have and then input the results in the Manual Entry of Spending Changes table.
While I hesitate to mention it, there is a similar free tool called ficalc that is more flexible in the number of off chart income or spending.