| Da'ath |
I'd make it a feat, instead. Looking at the polearm master archetype for fighter, you have a great mechanic you can adapt to a feat. Unlike the polearm master, however, your penalties would never be reduced by level.
Example
Short Haft (Combat)
A character with this feat can shorten the grip on his spear or polearm with reach and use it against adjacent targets.
Prerequisites Base attack bonus +3, Dexterity 11
Benefit As an immediate action, a character can shorten the grip on his spear or polearm with reach and use it against adjacent targets. This action results in a -4 penalty on attack rolls with that weapon until he spends another immediate action to return to the normal grip.