Some voices will be able to load, but if the S900 has any of those signature voices you're looking for (that are not on the S700).., you cannot load these into the S700. The S700 does not allow you to import sample data. If one of the voices edited on the S900 is an overlapping voice on the S700 then you're good.
The obvious answer you'll get from members here is to UPGRADE. However, if money does not allow (as you've stated) then you obviously have to get the best you can afford.
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.