I agree with Ian and would not personally try to extend the useful capacity of a fixing bath by any other means than a two bath system. Fixer is not that expensive and your images should be the primary concern.
At one time Kodak recommended the use of a hypo eliminator solution which consisted of hydrogen peroxide and ammonia water. The ideo was to oxidize any thiosulfate ion still in the emulsion to sulfate. Paradoxically such treated film had less archival permanance than untreated film. It appears that some of the residual thiosulfate decomposes and coats the silver grains with silver sulfide. Sodium sulfide is used to increase the permanence of microfilm. Agfa Sisain did the same thing by using a thiocyanate.