[gtthree]

There is no central channel on the base hifi.

The only way you will create a centre channel is either via full refit of hifi or HK. Which wil be expensive and give you a poorer result than installing an after market processor such as an after market processor such as the Eton DSP or JBL MS8.
Should you install either of these you will need technic harness and still need to run your own centre channel speaker wire.

I suggest you search the forums on e90 and f30 for other user uinstalls as they will give you a good idea of the work involved.

Personally I am using a technic harness, eton dsp, eton subwoofers and I have a HK speaker set for the woofers and tweeters. Although I may swap these out for focal krs100's but that is further work as they are not plug and play.