Como así
 

I always thought it meant, like this, but I was told it means, how come? If it is not used in an interrogative form, could it ever meam like this?

It probably depends on the speaker and the variation of Spanish being spoken. That said, all you need is "así" to say "like this."

Do it like this. = Hazlo así. / Hazlo de esta manera.

As far as I know, this is an expression from Colombia, which is becoming popular in other regions through telenovelas and social networks. Even some people in Mexico are starting to use it regularly.


As I've seen it used, it asks for an explanation or shows disbelief/disapproval.

- ¿Ya te vas? Pero ¿cómo así? Aún es muy temprano. = ¿Por qué te vas tan temprano?
You're leaving already? But why? It's too early still.

- ¿Cómo así que no fuiste a la fiesta? = ¿Cómo que no fuiste a la fiesta?
What do you mean you didn't go to the party?

- ¿Cómo así que cancelaron el concierto?
How come did they cancel the concert?


By the way, "cómo así" is never put together to mean "like this". As Tomísimo said, it's just "así". :)

El "¿Cómo así?" está sostituyendo el "¿por qué?"

People,

I think that the following is also typical to express disbelief: ¿Así por las buenas?

Cheers!

I wouldn't say it's typical, but it may express disbelief at how easily someone was convinced of doing something.

To do something "por las buenas" is willingly.

-- Pedro se va a casar conmigo.
-- ¿Así, por las buenas? (The speaker is assuming that there is a reason why Pedro shouldn't marry the other person willingly.)

-- Sí, voy a hacer ese trabajo.
-- ¿Así, por las buenas? (The speaker thought it would be harder to convince the other person to do that job.)