@Nick Lukey - Nick, you certainly can open jpgs in Sigma Photo Pro (SSP).
When I first got SSP I had already read all the negative comments about it and these cameras, and my idea was to immediately convert the files to TIFFs and import them into Lightroom or Elements 11. And that's how I started. But curiosity got the better of me, and I started playing with SPP, and found that, although clunky, it can do some things VERY well. There is a video on youtube where some guy shows that he can do some things better in SPP than in other major software. It's not perfect, by any means, likewise the Merrill cameras, but they and it do some things remarkably well. Whether that will satisfy you, and complement the way you work, only you can tell...