Support gems in ‘cast on…’ slots.

I can’t seem to find an answer to something that’s bugging me.

I am using ‘cast on shock’ to cast ball lightning. Can I add support gems to ball lightning somehow? Do I just stick them in the sockets for ‘cast on shock’ along with the ball lightning skill gem? Does that work?

I’m following a build guide that says to use ‘deceleration’ support gem so ball lightning moves slower. If I try and place that support gem in a socket of ‘cast on shock’ along with the ball lightning skill gem the game says the support gem doesn’t work with cast on shock, which is correct…. But that’s not what I want anyway, I want it to be supporting ball lightning.

I tried experimenting by trying with and without the support gem socketed and I couldn’t really notice any difference in the speed of ball lightning, which makes me think it doesn’t work.

Anyone got any idea?