First, many hardware RAID solutions (and all software RAID I know about) will let you add a single disk beside an existing set. You can think of a single disk as a one disk RAID0.
Whether or not you can add a disk to a RAID0 set depends on the RAID implementation. An individual hardware RAID card may not let you do it, some might. As for mixing and matching disk speeds, you're going to be limited by the 7200 RPM. It's hard to say exactly how much slower as it depends on the workload. It'll still be faster than a single 10K or a single 7200 RPM disk, but slower than 3 10K disks.
If you post more details about your setup (RAID controller, how many SATA/SAS ports you have, etc) I can give more specific answers.