There are four types of resources available in CA PPM: Labor, Material, Equipment and Expense. I'm going to assume that 'external expense' means "Expense" and not "Labor" since you use the word 'expense' and don't want to inflate hours.
Is my assumption correct?
If yes, then when you create these 'external expense' resources as "Expense", the numbers you enter for them will be considered Units and not Hours - you won't be inflating the hours needed by the project. If you needed 5000 hours of Labor, you'll still need 5000 hours - after adding your Expense, you'll need 5000 hours of Labor and 'x' Units of Expense.