While you could buy a dedicated macro lenses there are so many options with Canon, so there is a good Photos of the Year review which explains the many different options from extension tubes, teleconverters and diopter thingies. Simply put, he recommends getting a 1.4x teleconverter which takes your 70-200 F/2.8 to a 300mm lense that is about as fast as the 100-400.
And then getting a Canon 500D macro close up lense adapter in the 77mm range which focus up to 500mm (0.5 meters) away and it is only $140 vs.$1400 for an 180mm F/2.8 macro!
As A digital picture says, if you can afford the 100MM F/2.8 macro dedicated prime lense go for it. The 500D is sharp in the center and ok in the corners, but of course the dedicated is better. The main limitation is the relatively narrow range of focus of this lense adapter
CameraHobby.com has a good technical explanation. Basically most normal lenses have a 1:10 ratio. That is an option that is one inch in real life comes out as 1/10 inch on the sensor. If you get a 1:1 lense, then incredibly a 1 inch wide flower takes up the entire frame and is 1 inch wide.
The other parameter is the close focusing distance which is the distance from the sensor to the closest object that will be in focus. The 500D means it is 0.5 meters from the front (you have to of course subtract the depth of the camera and the lense too :-). A Canon 5D mark II with a 70-200 lense is about a foot long, so a 1.5 foot 500D means you can get objects about six inches from the front of the lense 🙂
The other option are a set of extension tubes by kenko, these just offset the lense. A 50mm extension tube attached to 50mm lense would give you 1:1. if you stack this with a teleconverter, that distance counts too, so a 100MM lense with a 50MM extension tube and a 1.4x teleconvertor gives you 1:1. Confused yet?
The last problem is depth of field, a 1:1 lense has depth of field in millimeters. Again, he likes the 500D alone on a 70-200MM lense. So if you are traveling, instead of carrying the 100-400, bring a Canon 500D, a Canon EF 1.4x III Extender and you don't need the 100-400 F/3.5-5.6 because you get a 140-300MM F/4 so about the same. As an aside the 1.4x alone give you a 252MM 1.4:1 macro lense alone, so a standalone TC isn't a bad idea either. The main issue with the extender is that it does increase chromatic aberation and you lose sharpness too, but not a bad tradeoff for being on the road.
Pricing-wise a 1.4x is $320 for the older (and less sharp II) and$500 for the new III (wow!) and the 500D is $150 and a Kenko extension tube set is$180 plus a \$10 bag.