There are a number of "foreign AP" subcommands found in CLI but there are no official documentations as of yet.  For obvious reasons, Cisco will officially "not"support non-Cisco LAP to their WLC.

Many years ago, there was a company called Airespace that manufactured a wireless LAN controller. Years later, Cisco bought that company and assimilated the controller.  When Airespace first launched the controller, one of the feature-sets was to be able to add a "foreign AP" (third party).  This was primarily geared to the wireless LAN switch, which was a 12 or 24 port device that ran the controller code, but the APs would be plugged into the front switchports.  The "foreign AP" would be plugged into the switch/controller and would be added to WLAN 17, which was reserved for third-party APs.

Basically, it was a method to pipe all third-party APs to one VLAN interface via WLAN 17.

Additionally, you could perform some monitoring and channel configuration of Orinoco APs and Cisco 350 Aironet APs.

I believe the capability has been mostly deprecated, though the commands still exist in the controller code.

What do you mean "expand coverage with wireless bridges"?

Wireless bridges can be used to connect remote networks, not expand coverage for clients.

So you may not be able to add them to the controller, but you can use them without problems in combination with Cisco APs to provide Client Access.

You don't really need to control the bridges with the WLC in order for them to work.

Perhaps you can tell us more about your design so that we can assist you.