This couldn't be reliably implemented to be honest. The problem is that you often don't know what a device's asset type is until you access it with a valid scanning credential.
If you're rescanning a device that's already in the database, you could link the device being scanned to an existing asset based on MAC address and get the asset type that way, but even this would only work for devices in the same subnet. If the device is another subnet, you would have to log into it with a valid credential to retrieve the MAC addresses.
I assume you know this, but you can map SSH credentials to IP ranges as well, not just individual IP addresses. (You indicate that you have multiple subnets though.)