Yes. You can upload a photo of any garment you want to see on yourself, whether that is a screenshot from a shop, a photo of something hanging in your wardrobe, or an image from a small brand that has no try-on feature of its own.
There is also a curated Featured Collection if you would rather start from something ready to go, but you are not limited to it.
The quality of the garment photo matters more than most people expect. Front-facing on a plain background works best, whether that is a flat lay or a clean shot on a model. Busy lifestyle photos where the garment is half hidden behind a bag or an arm give the model less to work with, and the result reflects that.
If you are photographing something you already own, lay it flat in even light and shoot from directly above. It takes a few seconds and produces a noticeably cleaner render than a photo of the garment crumpled on a chair.