Yes. You can add a passepartout (also called a mat or border) to a fine art paper print without ordering a frame. It is produced differently from the framed version, so it is worth knowing what actually arrives.
How it works
On a framed print, the passepartout is a physical sheet of acid-free cardboard with a window cut out of it. It sits between the print and the glass and holds the print in place inside the frame. Without a frame there is nothing to hold that cardboard in place, and it cannot be shipped on its own without being damaged. So when a passepartout is ordered without a frame, the border is printed into the print file instead of being a physical piece of cardboard.
What you receive
One sheet of fine art paper in the size you ordered.
Your artwork printed in the centre, surrounded by a printed border in the passepartout colour you selected.
No separate piece of cardboard.
The visible artwork is the same size it would be behind a physical passepartout, and the border has the same width and colour. The only difference is that the border is part of the print itself and cannot be lifted off.
How it affects the size
Exactly as with a framed passepartout, the artwork is produced one size smaller than the size you ordered, because the border takes up the space around it. A 50×70 cm print with a passepartout has a 40×50 cm artwork area, for example. The full size chart is in How does passepartout affect artwork size?
If you want a real cardboard passepartout
Order the print with a frame. Every framed fine art paper print ordered with a passepartout is supplied with a real, acid-free cardboard mat, cut to fit and mounted inside the frame.