Are you looking to add a grapple arm to your existing bucket or do you want to switch out the bucket for a separate grapple? Countrybumpkin shows a photo of what you need to add a grapple to an existing bucket and that is the least expensive way to go. A separate grapple (look at the photo in my avatar) requires that you either have a skid steer adapter (not standard on Kubota
L3400) or that you get a specially made grapple to fit the Kubota pin mounts.
If you need just occasional use of the grapple then adding a grapple arm to your bucket is easily the best way to go. Costs vary depending on whether you buy a kit from ATI or Add A Grapple (I think these are about 600-700 bucks now) or you can get something less expensive off eBay and do the welding/bolting yourself.
A basic separate grapple like the Markham 48 inch will cost about $900 delivered.
You'll need a set of hydraulic controls too which I don't believe come standard with an
L3400. That will cost you somewhere between $400 for rear remotes you install to about $800-1000 for a diverter valve or third function valve installed by a dealer.
If you can give us further details on what you are trying to do with a grapple and what your budget is we can try to steer you in the right direction.