The answer in brief is “yes, but it’s not recommended”. There’s a number of reasons for this. Although a RAID setup can be expensive to create and maintain, if you want the benefits of a solid state drive, it’s not recommended to pair it with a RAID setup based on mechanical drives. Doing this will slow down performance: it won’t provide the speed increase that solid state drives are known for, and will simply cost you more and give less space than additional mechanical drive likely would.
There are ways to make it work: you can use your SSD as a boot drive for your operating system, while storing your data on the RAID drives. This will provide a large speed increase at boot time, while still allowing your RAID setup to function the way it always has. In general, every drive in a RAID setup should have the same architecture, and the closer to identical they are, the better. But it is possible to leave an SSD outside of an existing RAID array and have the two perform separate functions. If you want to add an SSD to your RAID setup, that’s probably the best way to do it.
If you want to switch to using an SSD or SSDs, it may be worth replacing your existing RAID array. SSDs have no moving parts and are individually less likely to fail, and data can be recovered from SSDs in many of the same ways it can be recovered from HDDs. Multiple SSDs can also be placed in RAID together, if you so choose. Replacing your existing RAID setup is probably the most sure-fire way to get all of the potential benefits from an SSD-based setup. You can also use a single HDD alongside an SSD RAID setup for storing large amounts of non-critical data (such as movies you’ve purchased online), and rely on the possibility of data recovery should that lone HDD fail.
There are also certain methods of creating a hybrid RAID array. Some RAID adapters, such as the series 8 or 8Q, can perform read operations off of the faster SSD while writing to both the SSD and the HDDs in the array. This increases the number of read operations per second, and thus performance, without improving or reducing the performance of write operations. This will also preserve many of RAID’s normal data recovery benefits, allowing data to be pulled from any of the RAID HDDs as normal.