What do you mean? The antennas you can choose are only Cisco branded antennas. If they are different manufacture, it will not be in the list. You can try to choose something similar and also set the dbi for the antenna.
Go to Configure > Access Point > choose the AP of your choice > under Radio Interface choose 802.11a/n > under Antenna is External Antenna drop down box.
WCS provides the ability to enable or disable the use of specific antennas. All antennas are enabled by default. At least one transmitting and one receiving antenna must be enabled. You cannot disable all transmitting or all receiving antennas at once.
Pease check the below links which may be helpful for you:
