Possible as an (expensive) aftermarket modification. There are number of people that modify hybrid vehicles by adding piggyback battery packs that either charge separately or are integrated into the existing charging system - there's no technical reason why you could not do the same to the Empulse.
I suspect Brammo would rather create a well-integrated pack; for example, they elected to make the Empulse 10.0 instead of an Empulse 6.0 with an extra 4.0 kwh hanging off the top/sides. The 10.0 kwh battery likely has tapped out the existing chassis packaging in any case, requiring either denser batteries, a redesign for a larger chassis, or packaging battery packs in locations typically occupied by luggage.
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Here's what a hypothetical 20kwh well-integrated brammo might look like - pardon the hand-waving.
* larger chassis - think a touring bike. Much heavier pack -> increase frame weight, suspension weight, brake weight, weight weight weight.
* heavier / bigger pack - 10.0kwh pack is ~150 lbs, 20.0kwh pack is ~300 lbs. Figure total weight 650-750 lbs.
* suburban range +50%, highway range +75% - say 150 miles suburban, 85-110 miles highway (65-85 mph).
* 80% charge in 3 hours with a 6.6kw 220v/30a charger, 12+ hours with a 110v/15a charger.
* the kicker - $$$. Probably around $26k: $2k for the larger chassis, +$10k for the larger pack.
I don't know how much of a market there is for a $26k electric "touring" motorcycle that has a sub 100 mile freeway range. And if the market's small, prices go up further.
