When tritium (ordinary hydrogen with two extra neutrons in its nucleus) emits a beta particle (basically an electron) it turns into helium, and also emits an extremely small neutral particle called a neutrino. In principle, one could take a helium atom, an electron, and a neutrino -- and squeeze them together and get back tritium.
In practice you're not likely to get these three players together in one place for a long enough time to make this happen, but "reverse beta decay" does take place in nature under special circumstances. The intense pressures and temperatures of decaying stars undergoing gravitational collapse can combine protons and electrons together to create neutrons. Under some conditions this process can convert the remaining mass in decaying star to 100% neutrons (a "neutron star") with unbelievably high densities.
A form of reverse beta decay occurs in some unstable atoms, where the inner-most orbital electrons have enough of a probability of being found inside the nucleus, that they can interact with protons and convert them to neutrons. This is known as "electron capture" and frequently competes with beta-decay as a mode of nuclear decay.
The Sudbury Neutrino Observatory (SNO) is designed to detect a kind of reverse beta reaction in a large container of heavy water, where incoming neutrinos (particularly those from the sun) convert neutrons in the heavy water to protons. The rate of this reaction is quite low (tens per day), but SNO scientists are very patient people.
As a general concept, we are certainly able to turn atoms into other atoms that go quite against the direction of natural decay, simply by taking chunks of nuclei and throwing them at another atom with enough kinetic energy. The "chunks of nuclei" can be in the form of alpha particles, protons, neutrons, or other nuclei -- of any size. Particle accelerators and research reactors are used to for this purpose.
Even gamma rays (another product of radioactive decay) can induce nuclear reactions: a gamma photon emitted by the uranium fuel in a heavy-water reactor like CANDU can interact with a heavy water nucleus, kick out a neutron, and convert it light water.
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