3s to 4s battery conversion?

Ive been thinking about some up grades for my board… Im currently hitting 30kph but Im wanting some extra power
Im currently running 6s and a shitty hk esc (hopefully upgrading to a enertion vesc soon)
Im thinking about upgrading to an 8s pack or just buying another 3s battery but 9s is very uncommon.
Instead of buying two new 4s packs, I have been thinking if I can mod my 3s battery by adding an extra cell and a new balance connector.

Yes you can. Just keep the mAh the same.

ok does the c rating need to match?

It doesnt have to. But it would be nice.

You can buy one 1s battery and one 4s battery, and put the 3 batteries in Series, the you will get a 8s battery, but I don’t know the capacity of the new 8s battery, and what’s the weight of it ?