If you're using the analog 5.1 output (the 3 x 3.5mm jacks) you can get a
3.5mm Y adapter and use one for your current z906 and the other one (the sub output only of course) can be used for your new sub.
If you're using the digital inputs the only way is to open up the sub amp and tap in to either the input for low-level or the speaker terminals for high-level output, not really an option if you're not handy with a soldering iron.