Is it possible to mod a passive sub to make it a powered sub?

Yes, .. quite easy
you will need a sub plate amplifier (several choices available)

or
an old amp you may have lying around (assuming your reciever (if that's what you are using) already has a crossover built into it), ..

additionally, if your passive has a crossover built into it, . you will need to rip that out, .. since the crossover shaping in a power sub is already taken care of at signal (preamp) level

I'd check out hometheaterforum.com
Great site

It really depends if it actually is a speaker or not. Alot of speaker Manufacturers use just a cone and suspension, no coil involved. The passive radiator acts like a port without some of the negative effects of a port. If your radiator has a magnet and a coil, no problem.

Oftentimes the engineer designing the system factored in the crossover to the enclosure design. (The crossover introduces DCR, which changes Qes`, which changes Qts`, which helps determine how the driver interacts with the enclosure.)

But we are talking about a passive sub. It might be older than some of the posters at sounddomain, with more "profit" engineered into it than "great sound".

I say try it both ways. One way will sound better than the other.