You should be able to if you select the appropriate stream, and then start recording - but you'll have to leave it on that stream. You won't be able to do the usual sky+ recording features, i.e. watching another channel whilst recording BBCi, as the interactive stream will have logged off.
Basically, the HDD in the sky+ box is recording all the time, so will record whatever you are viewing. It should record a live stream from BBCi, but you won't be able to set the box to record a BBCi stream that airs later - you'll only be able to capture it whilst viewing it