You are not going to be able to use WiFi.RSSI() directly but you can modify that code (enclosed below) to meet your needs. You might also want to read the TI CC3000 documentation on wlan_ioctl_get_scan_results.
There is an outer loop with a timeout value and an inner loop that goes through the 16 possible RSSI values, looking for the one that has an SSID that matches the current SSID. The data is returned by the TI CC3000 in an array structure (wlan_scan_results_table) with a new value returned every time you call wlan_ioctl_get_scan_results and the return value is not zero, which means you hit the end.
The SSID is in the return value in array locations 12 to 42 (max) so you can see the code copies wlan_scan_results_table into wlan_scan_results_ssid. If the SSID matches ip_config.uaSSID, then that is our current network. This is the part you need to change to just use or copy all the SSIDs and RSSIs.
_functionStart = millis();
_returnValue = 0;
while ((millis() - _functionStart) < 1000)
_loopCount = 0;
while (_loopCount++ < 16)
unsigned char wlan_scan_results_table;
if(wlan_ioctl_get_scan_results(0, wlan_scan_results_table) != 0) return(1);
for (int i = 12; i <= 43; i++)
int arrayPos = i - 12;
wlan_scan_results_ssid[arrayPos] = wlan_scan_results_table[i];
// modify here to test for your network SSID string
if (*wlan_scan_results_ssid == *ip_config.uaSSID) _returnValue = ((wlan_scan_results_table >> 1) - 127);
if (wlan_scan_results_table == 0) break;
if (_returnValue != 0) return(_returnValue);