Hi Danni. I don't think mixing Farex to the bottle is a good idea. Don't worry, I've tried it, so I'm not just saying it out of no where.
I was desperate with Bri's colic & was willing to try anything to stop the screaming. MIL told me she used to put Farex in the bottle for DH to fill him up more. I did it for a bit - just about a teaspoon, but then I said something to my mum about it.
She said she'd seen people do it, but the ones she saw were the 'get baby to sleep longer so I can party' types. Which at 16/17 is the type MIL was.
They did it in younger babies too.
I don't think it'll be necessary. You're better off feeding it to her if you want her to have it.