Thanks for asking. But we currently don't support this option, sorry.
There is a work-around to this, while not optimal, is the way we recommend people accomplish what you want.
First, keep in mind that if a form has a $0 balance at payment time, then we will skip the PayPal processing step and send the confirmation email right away. So design your form as follows:
Add to your form a choice that asks if the user is paying by check or PayPal, which they have to choose before continuing into the payment fields.
Then, duplicate each of the payment fields on your form. Hook one set up to PayPal processing and do not hook up the others.
Finally, using skip logic rules, configure the form to show the PayPal fields if PayPal is selected and the check fields otherwise.
In this manner, if the user selects "check", they will be shown one set of fields that will not affect the dollar balance, and they will not be sent to PayPal. They'll simply receive the submission receipt email with your instructions. If they select "PayPal" they'll be shown another set of fields that do contribute to the balance, and that will result in a PayPal step.
The down side to this is that you're going to have duplicate fields in your results table.
Here's a sample form that I created to show what the workflow might look like. Note that I haven't connected it to PayPal, this is just to show the design.
Hope this helps,