if so, would it be possible to make (or remake) an organism that could subsist off of sunlight and external heat like plants?
Plants and algaes and cyanobacteria already exist. What exactly do you want?
No, there are organisms like that for instance coral, but in order for them to give us energy we would need to fuse are DNA with theirs
There are, in fact, protozoa that use chlorophyll to harvest sunlight, but the thing is, they also MOVE. So it's not entirely unreasonable that an animal cell could do the same.
It's done under the name of biodiesel. Plants capture sunlilght and produces oil, oil is used to generate movement by an engine. That can be used for transport or to generate electricity. But they can also get used more directly for solar panels: . though they aren't much good yet. Generally going to convert plant products into energy is still better. Edit: connecting a solar panel to your metabolism will remain for a long time science fiction. The traditional way of eating your food instead of absorbing it into your skin works well and can be very enjoyable.
