Well, it would definitely be qa bit of a hack and, as mentioned, it would only work if there are wires between the motherboard and speakers. Crack open the case, drill a hole to mount the new headphone jack and splice in connecting wires to it from the speaker's wiring.
Cheers,
Nigel.
P.S. I'm in a bit of a hardware hacking mood at the moment - I'm currently resurecting my old Samsung n145 (motherboard killed by an overvolting PSU) by replacing its motherboard with that from an n130 (£4 off ebay and all hardware supported by AROS bar, ironically, its speakers), and I'm taking the opportunity to add an uprated battery, supported Atheros PCIe wifi, touchscreen and mSATA SSD.