Only Rahmani development files are available from BIS, hence its the only original island you can edit, Visitor3 handles it obviously.
If you mean OFP islands, these are not very suitable for ArmA due to their older technique of contruction. There is typically problems with these OFP islands converted and running in ArmA.
The way I have converted my OFP island is to rewind to the raw data (height map, raw textures, object placement map) and basicially start from scratch in Visitor3.
If you have orignal OFP island data and want to try this, the experts at the PMC forums may be able to help.http://tactical.nekromantix.com/forum/