2 simple questions.

1. I have an activate/deactivate ability that increases range on a ranged unit
But I cannot find any appropriate icon for it (I need a default wc3 icon)
Any ideas?

2. Is there any other way to increase units range except using upgrades?

1: For this, I'd have to know what type of attack the ranged unit has.
2: Will the ability be upgradeable?

1. arrows
2. yes (you mean levelable)

1: Best I can think of that is default would be the Marksmanship upgrade icon.
2: Make 2 identical units (Except range) and switch between them when the ability is activated. Sorry, but that's the only thing I can think of right now.

Marksmanship look ok.
As for range upgrade:
http://www.wc3campaigns.net/showthread.php?t=85278

Upgrades only work one-way. If you want to switch between ranges you'll need to use some form of morph.

Does that mean that I cannot return level of upgrade to 0 once I set it to 1???

Yes, that's what it means. Upgrades are one-way traffic.

Exactly.

Than I got a big big problem.....

EDIT:

Or I can change the problem.
Anyone have any ideas for agility ranged hero passive skills?

We could have ideas, but it would be better to post how's that hero (its theme, description, picture of it, current abilities, some stats maybe)

see the thing is I need a passive ability that will apply to a whole class of heroes.

And the class description is : agility ranged.
Everything else is allowed.

I would give agility ranged units an Evasion ability, or a critical strike. Bowmen type people have major disadvantages in melee combat, so they learn to dodge well, OR I would make ranged units have a 10% X3 critical, and a 10% X.5 critical, since ranged units would miss more often than melee, as well as have a chance to deal a lot of damage.

You can go for critical strike (in this case, sharpshooter), it fits generally with your requirement, but I think this has been done many times.

Why not a passive ability based on Vorpal Blades?? Something like the unit fires the arrow with such force than the arrow can damage several units in line.

Level 1: up to 2 units receive damage
Level 2: up to 3 units receive damage
Level 3: up to 4 units receive damage

I like the worpal blades idea.
Can it be turned on/off ?