Well, this can be done, but not the way you describe.
The built in combos work the way they work, and they always will work that way. However, you could script a warp.
The question is, though, do you need to? What problem, exactly, are you having with the warps? Don't forget there's four blue tiles (A, B, C, D), four warp combo types (A, B, C, D), and four pages on the warp setup dialog (A, B, C, D). Triggering a B Warp Combo will use Warp B, which will send Link to whatever blue tile you choose.
If you give a more detailed description of your problem, we'll probably be able to help you.
(If you still want the script for a customized warp (which can ONLY use Blue Tile A), read below)
Code:
const int sensitivity = 4;
ffc script myWarp {
void run(int dmap, int screen) {
while(true) {
if(Link->X + 16 + sensitivity > this->X && Link->X < this->X + 16 - sensitivity && Link->Y + 16 + sensitivity > this->Y && Link->Y < this->Y + 16 - sensitivity) {
//do something
Link->PitWarp(dmap, screen);
}
Waitframe();
}
}
}