What does that mean exactly? Activate to gain the passive component? Or an ability that when cast mimics an attack? Or do you want the passive ability to function like it normally would but have an active component as well? -> So it'd basically be an active ability with additional passive effects.
In regards to mimicking an attack completely, it's quite difficult if not near impossible to do so. That being said, you could probably get pretty close to what you want.
For critical strike, you could use an Ability that does nothing, let's say Forked Lightning with it's Art/Damage/Additional Targets set to none. Then use a trigger to detect when this ability is cast and get the attack damage of the caster using the Get damage base/number of dice/sides of dice functions. Once you have the damage calculated and stored in a variable, apply the critical strike multiplier to it. So do something like Set DamageVar = DamageVar * 2.00. Finally, you damage the target using the "Cause damage to target" function and use the DamageVar as the damage amount. You can also create red Floating Text above the target displaying DamageVar to mimic critical strike.
This wouldn't be an actual attack but it's probably the most simple method out there. Keep in mind that there's other methods of doing this and that each method could vary depending on the mechanics of the passive abiity.
Also, just about anything that you can do in Jass can be done in GUI, Custom script IS Jass after all, so that's not really an issue to worry about.