This depends on the size of the drives in the array or whether your RAID card support SSD caching. A quick <insert your search engine of choice> should provide details on whether your card supports the latter. As for simply adding it to your existing RAID10, if the ssd is smaller than the smallest disk in your raid10 array, then it will not be used. In that case, it was initialize and simply waiting to be added to a new array. Also, keep in mind that RAID10 has to have even number of devices. You would need to add two devices or assign your spare to the RAID with the new SSD drive.