You need a reverse lens mount coupler, it looks sort of like a step up ring but there are threads on both sides, one for the thread for the filter size of the 100mm and one for the filter size of the lens you wish to attach. You have to match it to the filter size of each lens. I bought one, but then never used it. It still resides in my gadget bag for one day when I get the urge to do so.
One thing I never liked about some extreme macro, some people bring the insects inside, put them under a cup with high powered lighting or pin them down. I don't believe in killing insects to get my macro shots. Even the thought of it, turns my stomach and mind against people who do this kind of photography.
There are easy ways to get extreme macro in the field. Just get up early in the morning, before the dew dries, when they're still too cold to move.