This is certainly possible, but it's not something I'd recommend doing often.
The problem you'll run into is likely not having enough low-end, because your tops are going to be getting the same power as your lows. As a rule of thumb, the low-end is much harder to create, so you need more power for them, and you'll not benefit as much from having a sub in this case. It might be better than nothing, it might be worse (especially if that passive x-over fails and you end up with a full-range voice-coil!), but in any scenario it isn't ideal.
You can find powered subs that will take a powered signal. This will make sure the subs are getting the power they really need from their own amps, and you get the convenience of just jumping the tops right to them (no extra cable runs needed, or x-over splits, etc.).
"Knowing nothing is better than knowing it all."