Can you use the rules for improvised weapons and have a large item, such as a 10 feet pole arm to gain reach and threaten a larger area?
The DM can permit some improvised weapons to have reach
While the Player's Handbook describes improvised weapons (113), Complete Warrior on Improvised Weapons summarizes those rules and expands them, saying
As detailed in the Player’s Handbook, objects not crafted to be used as weapons may nonetheless find use in combat.…
Any creature using an improvised weapon—from a broken bottle to a barstool—in a fight is considered to be nonproficient with it and thus takes a −4 penalty on attack rolls made with it. An improvised weapon scores a threat (a possible critical hit) on a natural roll of 20 and deals double damage on a critical hit. An improvised thrown weapon has a range increment of 10 feet.
What about fighting with ladders, wagon wheels, or coils of rope? In the hands of a determined (or desperate) individual, these oddly shaped objects offer a wide variety of options in combat. … [one of which may be…]
Reach: Long objects (such as ladders) have reach, allowing a Small or Medium character to strike at opponents up to 10 feet away (but not at adjacent foes). These objects are treated as two-handed weapons. (158-9)
However, I suspect—as the question's example posits using a pole arm as an improvised weapon—that you're not thinking about bonking a dude with a ladder or the ubiquitous 10-ft. pole but, instead, about smacking a dude some distance away with your non-reach-yet-hafted halberd or spear. In such a case, first the DM decides if that object that's crafted to be a weapon can also be an improvised weapon then, second, the DM decides if the weapon-that's-an-improvised-weapon gains reach.
(The DM is wise to be wary of allowing objects crafted to be weapons to also be used as improvised weapons. For example, allowing weapons to function as improvised weapons means a crossbowman now threatens squares—and can make attacks of opportunity—by clubbing folks with his crossbow-as-an-improvised weapon, eliminating one of a low-level ranged combatant's primary weaknesses.)