Maybe…
There would have to be a new field added to the form. It would then need to be read by "ajCart/cart.php" (the shopping cart contents screen code), and then passed along to Paypal through a Paypal acceptable variable name. (found through the Paypal developer code guide).
It most likely CAN be done, we just haven't written the code yet for it.
(I don't know of anyone who's reported doing it.)