DDJ-SB + modular controller on Serato

Yes and it is quite simple actually. All you need is another USB port obviously.

Now the SB has to be connected first for Serato to recognise the authorised hardware, then simply plug in the other midi device and hit the MIDI button on the top of Serato. You can then easily map the additional midi device by simply clicking the function in Serato and the button you want to correspond on your device… very very easy. There is also a tutorial video on the Serato webpage.