If you could rig it to work, you would probably have to hook the passive to it in series to make it work. It probably wouldn't be pretty and you most likely would not like the result. You would most likely end up up with phase issues and most likely worse bass than with the single powered unit.
Yep, that would work. Also, you could use the Y connector with one active and one passive -- you'd just have to put a power amp in front of the passive sub.
If your receiver is flexible and allows you to still have a sub with your mains set to large, then you can connect the passive sub to the main speaker level outs, either in parallel or with two separate connections, and use the sub pre-out for the active sub (with the mains set to large, sub=yes, and LFE+mains) . The thing is, you will draw more current from your receiver this way because it will be handling lower frequencies driving the passive sub.
Other wise.. go right to the speaker of the powerd one, and do parallel to the other passive subs speaker.
Beware, this is only really acceptable, as long as your speakers are the same Impeadance, and the half impeadance doesn't go below the amp in the powered subs low rating. (not a very good chance, most subs have 4 ohm drivers, already at the lowest rated load for the amp)
A second amp is the better solution indeed, by far.