On my 2012 that,s what i did..

I added two ZF3 battery (18s2p cells only ) directly to the actual ZF9 with BMS.

What i did was to parallel every single connnections between the zf9 BMS and the two ZF3 as well as power connections that i engineered in a way that the currnet sharig is perfectlt and equal between the total cells.

i fact the ZF9 is a 3x ZF3 brick but with only one BMS for all 3.. what i did was to add 2 brick for a total of 5 bricks and get fron 120Ah to 200Ah all controllerd by the same BMS.

As well this simplify the digital communications and no need to understand the code that Zero use on the CAN lines or any analog lines like "Throttle engage" or "ON BMS signal"

But this require PATIENCE making the complete Harness and make sure it keep the system safe.

The 2012 are the only year to use 18s ( 66V nom) so i used some DB19 connetors, 20 gauge cable bundle wires and a splitter. Everything can be removed safely.

It work like a charm since 3 years now. cells remain perfectly balanced and are protected all by the same single original BMS.

I have used SB120 and 2 gauge wire that connect to the ZF9 pack inside and right on the main power connections but i aded a 300a fuse in line. Then these two 2 AWG wires split in a Y to dual pair of 4AWG wire then to a SB50 connector each pair to the two added battery brick.

I have measured the amp on the power connections and every added brick draw exactly 1/5 of the total current witch is perfect! 5 brick total each have 1/5 of current Awsome!

I also used dielectric grease to protect the DB 19 connections. and i also added a 150A fuse on each of the two added brick so in case of crash and hard disconnect or short, nothing will catch fire.

Now in spring 2017.. my plan are to do the same but with a ZF11.4 at 102V.. i'm a little tired of a powertrain driving at 66V ( 2012 model).. I want full power capability.. so i might remplace the ZF9 for a ZF11.4 and use two external Zf2.8. as well this will require replacing the entire wiring harness and DC-DC and MBB... mor eto come! Stay tuned!

take a look to my 2012 ZF15 ! : I have all pictures of the conversion here:

http://roulezelectrique.com/quebec-la-sarre-quebec-en-moto-electrique-2000km-pour-0/Doc