Is it possible for a star to return to the main sequence after entered in the post-main sequence phase? I have no clue if there's a mechanism such as acquiring hydrogen from from the environment or from a substellar body might bring again the star into the main sequence. Was it ever explored as a possibility?
The evolution of a star is irreversible process, so strictly speaking, it is impossible to return the star back to its earlier phase (it is impossible to collect all energy it has radiated to space and use it for conversion of helium back to hydrogen).
But it is possible for a star plotted on HR diagram to return to a point corresponding to main sequence phase. These stars are called blue stragglers. They are usually formed when stars merge.