Import Sony FS5 MXF into CS5.5?
You've hit the very unfortunate planned obsolescence issue.  Adobe only updates their software for a limited amount of time, so when working in RAW formats, you eventually hit a point where the old software simply will not work with the footage from new cameras anymore.

Your options are unfortunately a bit limited.  You can either update to a newer version of your editing software or use a different software for transcoding.  I'm not particularly familiar with Sony's workflow as I use an Ursa Mini (which uses CinemaDNG), but I'd suspect there to be at least a few options available for ingesting and transcoding MXF as Sony is a significant enough player in the space.

Looking at the documentation for Black Magic's DaVinci Resolve, it looks like you might be able to use that.  I can see from their documentation that MXF is a container format, so it's going to largely depend on what the video format within the MXF is, but DaVinci Resolve has a pretty long list of supported MXF codecs.  If it does support it, Resolve has a free version that could likely be used to do your transcode for you.  In the beta of Resolve 14, you can simply add the files to a project, go to Media Management (under the File menu) and it will let you transcode all videos in the project to another format of your choice.