Yes you can use third party cameras with the HDX 7000. If you are going to use one you might want to look into getting this cable
This cable breaks out the HDCI camera cable video and control signals to
standard interfaces. This cable can be connected to the EagleEye View camera,
but does not support audio. The five BNC connectors can be used to carry
composite video, S-Video, or analogcomponent YPbPr video. The DB-9
connector is used to connect to PTZ camera control interfaces. It is male HDCI
to five female BNC and one female DB-9.
The supported cameras are located in the HDX release notes for the version you have, in the interoperability section.
The release notes can be found here
Once you have verified your camera is supported you can talk to your Polycom sales consultant to get the right cable, which maybe the one quoted in the previous post.
Others may work, but only the ones listed in the release notes are supported.
Also be aware that with the HDX 6000, 7000 and 8000 the only IR receiver for the remote control is located in the camera. If you use a camera, supported or not, without an IR receiver you must control the system via the API.