You can definitely create a Fusion Drive using an external drive to add capacity, and in fact it can be a great thing to do as the SSD will eliminate most of the latency involved.
The SSD will contain whatever the most frequently used blocks are, and these will moved from your HDD(s) onto the SSD to provide the performance improvement. This means you could potentially end up with any number of your working files on the SSD depending upon how often you use them (or have been using them recently). This is important to keep in mind; the SSD in a Fusion Drive is not the cache as a cache-disk, meaning that if the SSD fails you will lose data and have to rebuild your Fusion Drive from backups.
This will essentially eliminate the redundancy advantage of RAID-5, as while you can replace any HDDs that fail as normal, an SSD failure will render the volume unusable.
I'm actually currently interested in whether a RAID-1 of SSDs can be used within a Fusion Drive, as it could allow the SSD part of the Fusion Drive to be given redundancy too, in which case the setup would be safe as it'd be 2x SSDs in RAID-1 + HDDs in RAID-5, giving you one to two disks worth of redundancy. However I suspect it won't work, as I believe Fusion Drive just queries each disk to determine if it is an SSD, and of course an Apple RAID won't identify itself as such.