If you need more channels you can alter the firmware on the FSi6 to give you 10 channels then use a Flysky multi channel receiver.
If you just use a multi module then you won’t get more channels. You would still need to change the firmware. This has some risk of bricking with it, I haven’t done it myself so couldn’t comment on how effective it is. The Flysky TX doesn’t have a module bay either, so you will have to directly wire any multi module.
What particular features of the X8R do you need vs a Flysky one?
In short, no. Flysky uses AFHDS while FrSky uses ACCST. AFHDS and ACCST are the frequency hopping protocols used by the transmitter and receiver so that they can talk to one another. In effect, the transmitter is speaking a different language than the receiver can understand, and as such binding the two is impossible. However there are multiprotocol modules available for this model of transmitter which will make it be able to use the receiver you mentioned, so if you already bought it not all is lost.
To me, it looks like you have 3 choices, get a new Rx, get a new Tx or as @speedbirdted suggested, try a multi protocol module. I’m not sure if the module will work in your Tx.
I use the banggood multi protocol in my Tx. I primarily use Flysky Rx, I have bound to DSM2/DSMx, I have not tried to bind to Frsky. I’m very pleased with the performance, I’ve never had a range issue due to the module.
It’s unfortunate that the two brands names are so close.