The most logical method of replenishing the helium would be by the natural alpha decay of a radioactive material such as uranium. But since the telescope has used rather a lot of helium in the four years of operation, you would need an awful lot of the stuff in the first place, and have it at the Lagrange L2 position beforehand.
The other option would be to use something other than helium for the cooling process but ten years ago it was really the only option, again for the 'all in one package' launch scenario..