Is it possible to mock a function imported from a package in golang?

Yes, with a simple refactoring. Create a zSomeFunc variable of function type, initialized with z.SomeFunc, and have your package call that instead of z.SomeFunc():

var zSomeFunc = z.SomeFunc

func abc() {
    // ...
    v := zSomeFunc()
    // ...
}

In tests you may assign another function to zSomeFunc, one that is defined in tests, and does whatever the test wants it to.

For example:

func TestAbc(t *testing.T) {
    // Save current function and restore at the end:
    old := zSomeFunc
    defer func() { zSomeFunc = old }()

    zSomeFunc = func() int {
        // This will be called, do whatever you want to,
        // return whatever you want to
        return 1
    }

    // Call the tested function
    abc()

    // Check expected behavior
}

See related / possible duplicate: Testing os.Exit scenarios in Go with coverage information (coveralls.io/Goveralls)

One thing you can do is this:

import "x/y/z"

var someFunc = z.SomeFunc

func abc() {
    ...
    v := someFunc()
    ... 
}

And in your test file you would do this.

func Test_abc() {
    someFunc = mockFunc
    abc()
}

But make sure that you do this in a concurrent manner, if you have multiple TestXxx functions calling abc or setting someFunc you may be better of using a struct with a someFunc field.