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Basic RAID controllers that come in the form of simple adapters in servers usually don't provide the ability to dynamically grow a RAID array. However, more intelligent storage subsystems do have this functionality. I'm talking about storage systems like IBM's DS-series storage, EMC Corp.'s CX-series storage, Network Appliance Inc.'s FAS-series storage, Hewlett Packard Co. EVA-series, etc. These types of storage systems allow the dynamic inclusion of hard drives. These systems also either allow the data to be migrated from one RAID group/type to another (eg., 3-disk RAID 5 to 6-disk RAID 10) or dynamically added to an aggregate of drives that define a RAID group. The re-striping process occurs behind the scenes to spread the data across the existing and the new drives.
However, these are enterprise-class storage systems that are in the order of tens of thousands to hundreds of thousands of dollars and usually are part of a larger SAN. There are some lower end storage systems with some dynamic functionality such as EMC's AX150 and HP's All-In-One storage systems, but even these are in the upper thousands range.
So, basically your idea of growing your simple RAID group by dynamically adding drives is unlikely unless you've managed to find a RAID controller with exceptional functionality, which is entirely possible. Given today's state of the art, I've taken a lesson from Sean Connery and learned never to say never.*
*For the uninformed, Sean once said that he would never play James Bond again, only to do so many years later in a Bond movie aptly named Never Say Never Again.
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