Actually, there is a semi-simple way to do just that.
It involves using some codes, an edited .rel file, and taking the Marth/Roy fighter folder contents and putting them into the folder of whomever you wish to replace.
The only catch is that there are some graphical hiccups if both movesets are present in the same match, and SFX issues.
As for things like their portraits and character select icons and whatnot, those would need to be replaced in the common5.pac and a few other places.
I actually have all of the things needed to .rel port Marth/Roy movesets in my Roy PSA download, as well as things like CSPs, and whatnot.
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=29268I can help you with it if it's too confusing.