JNGP maps are openable in regular WE. It will always bring an error up saying that "Note: This map has been saved by an older version of WE" but that isn't really anything, you can just ignore it. The only time that it will "protect" the map is when you use the custom trigger functions that are enabled through UMSWE or whatever. (or when you use vJASS, but that isn't protecting the map. That will just throw an error if you try to save it in regular WE)
And editing the tileset is fine, you won't have any problems with opening a map that has more tiles. You can just use JNGP to add the tiles, then switch to the regular WE and continue mapping.