If statement for ffdf object

I have an ffdf object x that holds a dataset with variables y and z. Due to millions of rows it needs to be stored as an ffdf object. My question is this:

I want to create a new variable within this object q, that is dependent on the value of z.

z takes three values: 1, 2, and 3, and when z=1 I would like q=A, when z=2, I would like q=B and when z=3, I would like q=C.

In reality z has about 30 levels, and I have tried the ffifelse statement which seems very inefficient. I have also tried the following:

x$q[x$z=1] <- ff("A")

but I then get the following error:

Error: length(value) == n is not TRUE

I would appreciate any help on transforming and creating new variables within an ffdf object.