A coaster brake is part of the rear hub - it's not something you can add to an existing hub. So, realistically , you'd need to source a new rear wheel built around a coaster brake hub.
Most geared bikes have 130 or 135mm wide rear hubs (and the frame spaced to suit); single speeds (with or without a coaster) are often (but not always!) 120mm wide. The bike you're looking at may have 120mm rear spacing, or it may be 130mm. It's possible - but a bit of a bodge - to put an extra ~5mm of spacers on either side of a 120mm hub to make it fit a 130mm frame, although that can mess with the alignment between the front chainwheel and rear sprocket (they should be lined up perfectly to make sure the chain doesn't jump off).
What Marx was saying about the rear drop-outs is valid. The bike you linked to has forward-facing dropouts. They would work with a coaster brake, although rear-facing would be better.
Sourcing a coaster-brake wheel... that might be a bit trickier. Obviously there's no shortage of kids bikes with coaster brakes, but fiinding one built up to a 700c (adult road bike size) rim would be a rarity. edit: the BigW ad says it has "66cm" wheels. That converts to 26", which is standard mountain bike size... smaller than 700c which is standard road size. That's not going to make it any easier to find a cheap off-the-shelf wheel.
I've got one (a 3-speed coaster brake hub, no less!), but you can't have it
. Your friendly bike shop might be able to help, but it's not something they'd have lying around... worst case, you could get one built.
So... technically, no problem. Can be done. Logistically, might be a bit tricky. Tricky can be overcome, at a cost.
 just in terms of cost. Technically, it would be possible to disassemble the wheel and re-build the rim on to a coaster hub... but the labour to build a wheel locally would be comparable to the price of the entire bike you're looking at.