FLV is not intended for editing. Flash may be the better application to import into.
In most cases, it should work fine, without the need for any additional codecs. I've successfully imported FLVs containing both Sorenson Spark and On2 VP6-encoded video, and MP4/F4Vs containing H.264-encoded video. I've had no issues doing basic editing on these sources and exporting them to a new destination, with the obvious caveat that you're (usually) dealing with highly-compressed video, and the quality will drop in varying degrees.