Is a suspension needed to do wheelies?
A wheelie is riding along on your rear wheel, using the torque from pedaling to keep the front wheel up. 

A manual is rising along on your rear wheel, coasting and shifting your mass backwards behind the rear wheel to keep the front wheel up.

You do not need suspension to do wheelies or manuals. You can find an enumerable amount of videos of people on YouTube doing wheelies and manuals on full suspension mountain bikes, hardtail mountain bikes, and even fully rigid mountain bikes, trials bikes, and BMX bikes.

Generally speaking, you don't add suspension to a bike that didn't start out with it. Rear suspension has to be designed into the frame. Suspension forks can technically be added to almost any bike. However, frames that were not designed for front suspension will have their geometry altered and likely aren't capable of handling the forces that come along with riding that requires suspension.