Certainly! You don't need to abandon your characters just because they weren't created in the TCG.
Though it's not quite as convenient as importing a character straight from the TCG, it's easy to enter your character's information manually. You can even upload an image for them (3D models currently only work with TCG-generated characters, sorry). Everything is editable, so don't worry if you want to change something later, or you make a typo.