Adding a new field to Potentials

Adding a new field to Potentials

I have just signed up for the free 3 user licence.  It appears as though I am restricted to edit fields and cannot add new fields, is this correct?

Potentials show a field for contract revenue.  I would like to have 2 revenue fields, one for services and the other for licence fees.  

If I upgrade, can I make such a change?