You may need to use wire cutters to remove the soldered jump ring that comes on the triathalon oval (sometimes it's too small to fit over the chain end).
Use a new jump ring to attach directly to necklace chain. For best way to open and close loops, see Jump Ring Hint in Jewelry Basics 101 (PDF)
Use a second jump ring to attach a Swarovski Channel Crystal Charm in a color of your choice. (To see other colors click into "Related Products" tab of the crystal charm below.)
1 each #88-850-05 Nina Designs Sterling Silver Triathlon Oval Charm
1 each #46-590-14-042 4mm Cubic Zirconia Sterling Channel Charm - Diamond - or - 1 dozen [use 1] #46-398-1-01 Swarovski Channel 29ss 1-loop Round - Crystal/ Rhodium Plated (currently out of production ... but we're working on getting someone to make them again!)