Only a 6' spear though. A 12' spear is just too unwieldy due to its excessive length. Also a jo, like a quarterstaff does not generally taper, unlike, say, the Greek hoplite spear (though not all spears did taper in such a manner to be fair).
Staves, including oriental ones, tended to be at least 25% thicker in diameter than a spear of equivalent length. This does not mean a 6' spear could not be used like a staff in combat, but it would necessarily be more fragile and do less damage (on the whole, there would be the odd exception given the natural variation of these weapons).
Edit: for clarity, as it strikes me we may be slightly at cross purposes, I'm aware some staves, including some (though a minority) of oriental ones do taper, they do so at both ends and are still generally thicker than an equivalent spear. The spear tends to taper at the 'working' end only and will start to do so from a fair way back down the shaft - to allow the balance point to be further back, improving reach (and the whole point, excuse the pun, of a spear is to have decent reach), especially for the longer spears. Making a good spear shaft required some craftsmanship.
