I think that the answer is, maybe. It will depend on the design of the portable. To figure it out, you would have to either get a schematic, or figure out the basic design of the circuits between the dac and filter, and the headphone jack. I would suggest just using the headphone out as the line out, and put some money towards a better source. But if the urge to mod your player has bitten you, you could do it as an experimental hack. If you can locate the opamp that drives your headphone jack, you could get the pinout online, and solder a cord in temporary, to jack into your headphone amp, to test it's operation as a line out.