1) you would lose the current data on the RAID0; meaning you would have 're-format' the raid0; losing all data currently on it
2) it won't add to performance; as the SSD's low access times will be raped by the HDDs access times; meaning any performance benefit is close to zero.
3) it won't increase the reliability of the RAID either; since RAID0 stores segments on the vulnerable HDDs.
An SSD is very much the opposite of what a HDD is:
HDD focuses on mass-storage with low price per GB
SSD focuses on performance with only limited storage space, much like SAS 15k HDDs.
HDDs are fast on sequential transfers, but slower than a turtle when involving random I/O. SSDs may actually be faster with random I/O than sequential I/O, depending on the degree of intelligence of the SSD controller. So actually sequential I/O is a bottleneck for the SSD, while random I/O is almost as fast or in some weird cases even faster. An SSD can be many hundreds of times faster than HDDs regarding random I/O.
So just use HDDs for mass storage, and use SSDs as system drive running your operating system. That's pretty much how the technology was meant to be used; not to completely replace HDDs as for mass-storage HDDs are still the better choice given a budget constraint.