I understand your original idea, but simply adding to the side will still leave the verticals flapping in the breeze (so to speak). That frame is designed to fit snugly into the boat with the verticals tightly against the tubes. leaving gaps could be dangerous and would be a significant waste of space.
I really think those are one of the toughest frames to modify but it can be done. If you look closely at the picture, that frame was built as a take down and the middle of each of the cross pieces is it's own section with an internal sleeve connection. I think that would be the only solid way to widen your frame. If it's not a break down you would need to make it into one.
You'll need to cut an equal amount out of each of the cross pieces, then replace those with longer pieces that make the frame the width you want. I'm pretty sure they use 3/4" EMT to make their frames - you could call them to find out. Either way you'll need to find some steel tubing that fits inside your frame tubing and make sleeves. You'll also of course need to get additional frame material, what ever that turns out to be.
If you can weld, great; weld the sleeves to one side of each piece (welding them to the outside pieces makes it easier to adjust size again in the future). If not JB weld or even heavy rivets, maybe even bolt it together (what ever floats your boat). You could then drill and pin/bolt/strap the inner pieces or do nothing and let the boat hold your frame together... I'm not quite sure how Recretec does it on their production frames but I believe the pressure fit of the frame in the boat is what keeps it one piece. Again, do what ever your comfortable with here...I know I wouldn't want a dozen or more pins to keep track of, so I'd either bolt it or let it pressure fit if it seems solid.
You can still make side decks but you'd need to get creative around the towers... My old recretec frame like that (or somebody's knock off) didn't have the inner support (long curved one) so side decks weren't a problem. but I'd think you could still make something that would fit just inside that support if your frame has it. Possibly even be notched to fit around it...
Hope that helps...
"If you dont do it this year, you will be one year older when you do"