to get improved knockdown i need 13int is it possible to use an item to get it to 13 and chose improved knockdown anyway and if that is possible,do i need to have 13int to use it?

No, it needs to be a natural 13 Int.

no, feats relying on attribute scores must be met unless it is offered via a class (i.e. Ranger's two weapon fighting feats or Monks Stunning Fist or Knockdown).

BTW, Knockdown is over-rated (monsters in NWN's are uber-powerfull with it but your character won't be, this problem was fixed in SOU for SOU only - presumably it will be fixed for other modules as well).

I used knockdown, and improved knockdown a lot in NWN, and well. It's important to remember that a halfling can't knock down a dragon, but could get maybe a hill giant, as creature size is a variable. therefore, a dragon can knock you down a lot.

Mages and even priests are easy to knock down, so it's a good way for a warrior to prevent them from casting.

im going to buy SoU and im playing a Dwarf so is Improved Knockdown worth it
and if it is,what stats should i lower to aford 13int i have 8 originaly

/sorry about bad speling

Lower wisdom or charisma (though you are at a disadvantage with charisma with a dwarf anyway).

Most creatures have a low Discipline skill so knockdown can be a great feat to have especially if you use it repeatedly against casters, disabling their spellcasting opportunities.

Sure - humanoid spellcasters can be disrupted from casting with a succesfull knockdown. However in the same time it takes to achieve a knockdown you can usually kill that spellcaster simply by hiting it repeatedly (and if you have a high number of attacks you will usually disrupt their spells b4 they ever cast a single spell). (Spellcasters typically run low on hitpoints, the ones that are easy to kill quickly, and the ones that don't usually are either immune to knockdown or are practically immune to knockdown.)

If your fighting melee style the key to just about all encounters is: hit hard (high damage), hit a lot (many attacks), always make contact (high hit probability).

Knockdown work even if the guy has Stone Skin or other protection spells on.