I'm making a strawberry shortcake this weekend.  I've been making it the same way for 15 years.  I thought that this time around I would try to add some FLARE to is and do roses around the cake.   Is there a way to stiffen cool whip?  Can I mix powdered sugar into it like I would for butter cream?

I don't think cool whip would be stiff enough for roses even if you added powdered sugar. Although I've never used Cool Whip so this is just my assumption. I recommend making buttercream or royal icing roses and just using those. They are both more predictable and reliable for your cake. If taste is an issue, just remove them before serving. 

I did an ice cream cone cake that had a cool whip "whipped cream" around the cone layer. It's a pudding frosting that looks like whipped cream, but is stiffer, so you can pipe with it and tasted yummy. Check out my photos and pm if you want the recipe. 

Maybe try Cool Whip's new frosting. It might be stiff enough to pipe roses