That's what stars do all the time.
For example, in the sun (and MOST other stars), deep down in the center
of the sun's core, two atoms of Hydrogen get squashed together so hard
that they blend into one atom of Helium AND release some energy.
That's where the sun's energy all comes from. It's called "nuclear fusion".
It needs tremendous temperature and pressure to happen. We know how
to do it, but we can't control it. So far, the only thing we've ever been able
to use it for is Hydrogen bombs.
There are 92 elements on the Periodic Table that are found in nature,
plus another 20 or so that have been made in the laboratory, but only
a few atoms of them.