I can't say about the Marumi but I used to use a Canon WD-58H 0.7x converter on a Sony DSC-F707 some time ago. It wasn't perfect but was quite useable. That was 58mm too and just screwed on to the front of the lens.
It's not as good as a wide angle lens on a DSLR, but if you don't have that choice then it's way better than nothing. I'd guess the Canon would be better quality and it might be worth having a look around for one on eBay.
