Does Kotlin Has Extension Class to Interface Like Swift

Can a Kotlin class be extended to conform to an Interface (like Swift classes can)?

Kotlin doesn't have this language feature, extension functions and extension properties are as far as the extension features go - these are available because they can be implemented via static helper methods that just get the receiver of the extension as their first parameter.

However, interfaces can not be added to existing classes that you might not be compiling yourself - that feature would be more than syntactic sugar. (On the JVM, at least.)

Can i make extensions in Kotlin as in Swift

Yes, Kotlin allows the use of Extensions.

fun Int.squared(): Int{
    // Your code
    // use `this` parameter to get int value
    // return value
}

and the use it anywhere like this:

val squared = 3.squared()

Import statement will be automatically included if you are trying to
use an extension

Kotlin equivalent for extensions and reduce

Kotlin does not support extending a class to retroactively add an interface. See this answer for example.

The Kotlin equivalent of Combine is Flow. It does include a reduce operator.

Not able to use Kotlin Extension Function written in common in iOS as Swift Extension

You're doing everything right here. Unfortunately, this option is not available for now. Extensions conversion may be performed correctly for some classes, but Swift's String is not the one. This is a known inconvenience, and K/N team is working hard to make it better.

Conditional interface in Kotlin

If I understand correctly what you're trying to accomplish, you can use an extension function with multiple types as upper bounds of the receiver type:

fun <T> T.presentAlert(message: String) 
    where T : UIViewController, T : AlertPresentable {
    // ...
}

Does Kotlin support or have future plan for Interface Composition similar to Protocol Composition in Swift?

You can not explicitly define intersection types in Kotlin, but you can use generic type constraints to achieve it in a function parameter. Like this:

interface Named {
    val name: String
}

interface Aged {
    val age: Int
}

fun <T> wishHappyBirthday(celebrator: T) where T : Named, T : Aged {
    println("Happy birthday, ${celebrator.name}, you're ${celebrator.age}!")
}