For less cost than the $10,000 Lithium option some people ( such as uhtrinity )have modified other HEV battery packs like those from wrecked Civics as booster batteries for the Insight.
Details are in this Thread:
2004 Honda Civic Battery:
http://www.insightcentral.net/commun...php?f=4&t=7411
For example the additional ~12Ah ( or ~1,700 Wh ) of batteries I got by buying two civic packs cost me ~$500... Some additional costs for electronics and such... but such a basic system is doable for ~$1k... same concept as the $10k... just requires you to do the work and has a little under 1/3 the range of the lithium $10k option.
If you have money to spend... the lithium option has benefits ... an example of someone doing a Lithium conversion themselves is here:
PHEV Insight:
http://www.insightcentral.net/commun...php?f=4&t=7464
The benefit of what GPSman1 was suggesting is that it has all the complexity done by someone else... so that you do not have to know anything about doing it yourself in order to get it done.
Details about the $10k option are here:
Plug-in Lithium Battery Insights for SALE!
http://www.insightcentral.net/commun...php?f=4&t=7964