I want to use my wifi cable modem router as a wifi router, no cable.

> [...] An ac750 dual channel WiFi extender [...]

   Not an actual model number (which would be more helpful).

> [...] can I connect my WiFi modem router the same way? [...]

   Possibly, more or less.

> [...] I'm worried about damaging something. [...]

   It's approximately impossible to do damage by connecting Ethernet
ports.  Many arrangements won't work, but actual damage would be hard to
achieve.

> [...] Does it matter which Ethernet Port I use? [...]

   All the Ethernet LAN ports on a C6300 are equivalent.  (The C6300
doesn't have an Ethernet WAN/Internet port.)

> [...] Can I turn off or disable the Modem? [...]

   You can ignore it.  You won't be using the co-ax connector.

> My current setup is an expensive kluge and has no Ethernet ports
> available.

   Depending on exactly what you have, a simple/cheap gigabit Ethernet
switch could be used to connect multiple devices to one Ethernet LAN
port on a router or similar device.

   It should be possible to configure your C6300 as a wireless access
point.  In that mode, you could connect your Powerline port to a LAN
port on the C6300, and then its Ethernet LAN ports and wireless LAN
radios would act as extensions of the main-router LAN.

   There's a step-by-step for configuring a C6300 this way here:

      https://community.netgear.com/t5/x/x/m-p/1463500

Some fiddling with the (unspecified) main router could be helpful in
that procedure.

   Knowing nothing about your "ac750 dual channel WiFi extender", I'm
not sure how it would fit into this scheme, but if it had been connected
to the Powerline gizmo, then you should be able to connect it to an
Ethernet LAN port on the C6300, instead.