While not described in this HOWTO, some useful tools for Software-RAID systems have been developed.
It is not easy to add another disk to an existing array. A tool to allow for just this operation has been developed, and is available from http://unthought.net/raidreconf. The tool will allow for conversion between RAID levels, for example converting a two-disk RAID-1 array into a four-disk RAID-5 array. It will also allow for chunk-size conversion, and simple disk adding.
Please note that this tool is not really "production ready". It seems to have worked well so far, but it is a rather time-consuming process that, if it fails, will absolutely guarantee that your data will be irrecoverably scattered over your disks. You absolutely must keep good backups prior to experimenting with this tool.
Remember, RAID is no substitute for good backups. No amount of redundancy in your RAID configuration is going to let you recover week or month old data, nor will a RAID survive fires, earthquakes, or other disasters.
It is imperative that you protect your data, not just with RAID, but with regular good backups. One excellent system for such backups, is the Amanda backup system.