Yes, you can!
Many of our fonts support these characters, which you can cut and paste into the text entry box after clicking the "Add text" button in the design studio. You can also create this text in another piece of software, the upload that file to our design studio using the "upload art" button.
Please note: These non-standard characters will sometimes look a bit off in the checkout preview images - but this is normal. What is actually printed will match the design you create in the design studio.