Pretty sure to do this, you could try following these steps:
1) Figure out which polygon is Captain America's Shield by either selecting the different polygons in the "Objects" folder and previewing them(Newest version of BrawlBox only), or by previewing the model and on the left pane, make invisible all polygons, and then check each one, until you find the shield.(All versions)
2) Change the name of the bone that the shield uses to match the name of the bone for Ivysaur's bulb/leaves, then export by selecting the polygon in the "Objects" folder, and right-clicking, and say "Export".
3) Open up the Ivysaur model and under the objects folder, select the bulb/leaves polygon, right-click, and "Replace" it with the new one.
4) Preview the model; on the left pane, select the bone that is the bone for the back, and on the bottom pane, you can change the "Transform" and "Rotation" of it, which is where it is located in space and what angle it is, respectively. This will ACTUALLY move it on the model itself. Then save.
This is written like this in case anyone else wants to know how.(Assuming it works, which it should); if it doesn't work, please tell me.
3) Import
It says an unhandled exception has occurred in my application. Insufficient memory to continue the execution of the program. I have no idea what that means but I'm assuming it should be obvious. This is whenever I try to replace it, even after I changed the bone names and whatnot. Should I just try to find a different shield? Lol. I'm using the Link one. But I figure it doesn't actually matter which one I'm using since they should all function relatively similarly.