Not with the existing filesystem. If you could put the data in a temporary spot, you can create a raid 0 array to make the two drives look like one big drive. If one drive fails, everything is gone though.
If the data is important, I would consider buying two drives. You could create a degraded raid 5 with the two drives, copy all the old info to the new raid, and then add the old drive to the new raid. This would allow one drive to fail without losing anything. It would also allow you to grow the raid 5 by adding more drives.
omv 0.6.0.8 kralizec | 64 bit | 3.12 backport kernel | omvextrasorg 0.6.16
omv-extras.org plugins source code and issue tracker - https://github.com/OpenMediaVault-Plugin-Developers