Currently, there's no direct support for FortiNet devices. You have the ability to add them as 'Universal Network Devices' in AirWave, but that only gives you monitoring ability of the FortiNet devices (and only if FortiNet devices respond to MIB II OIDs). It'd be a feature request to officially add support for FortiNet devices. You'd also want to alert your sales rep so that they can try to discuss with Product Management.
Details to add to any feature request: The FortiNet device models and firmware versions. Number of FortiNet devices present (and number expected if there's planned growth).