Yes, as the other answers have noted, "accumulatory" is definitely a word. What I'm not so convinced about is that it's the right word for the purpose you want to use it for. For that matter, I'm not even sure that "fragmentary" really makes sense in the context you're using it in, either.
The problem with "fragmentary" is that, at least to my ear, it describes something that is broken into pieces, scattered and/or incomplete. Specifically, it describes a state of being, not an action such as a collision. So one could, idiomatically, speak of an ancient "fragmentary document", of which only some parts survive, or, in astrophysics, perhaps of a "fragmentary body" that has been shattered into pieces by a collision. But I, at least, would not idiomatically describe the collision itself as fragmentary, since it's a single event that is not, in any meaningful sense, incomplete or consisting of multiple pieces.
(A possible exception, where "fragmentary collision" could seem appropriate, is if one of the colliding bodies was already broken into fragments that each separately struck the other body. In that case, one might indeed describe the collision event itself as involving multiple fragments.)
So what words should you use instead? Well, for collisions that break things apart, there's the perfectly good English word "fragmenting", derived from the verb "to fragment", i.e. to break something up into fragments.
Conversely, for its antonym, one might perhaps choose "accumulating", or possibly "agglomerating", "conglomerating" or "coalescing". Of course, the corresponding Latinate adjectives "accumulative", "agglomerative", "conglomerative" and "coalescent" would also work, although they don't parallel "fragmenting" quite so nicely.
Ps. I did some Google searching to try and find out what, if any, actual established terms of art there might be for these concepts.
Searching for "fragmentary collision" or "fragmentary collisions" does turn up a couple of seemingly relevant sources that use the term, although in some cases (such as "the fragmentary collision of Comet P / Shoemaker-Levy 9 with Jupiter") it's clearly used in the sense I noted above as the "possible exception", i.e. to describe a previously fragmented body colliding with another one. There are also a number of clearly irrelevant matches, such as "romance is a fragmentary collision of wary strangers".
However, "fragmenting collision" and "fragmenting collisions" both turn up orders of magnitude more results. A considerable fraction of them are clearly from astrophysical literature, although notably, the term also seems to be used in other fields such a physical chemistry or materials science to describe collisions between molecules or small grains or droplets of matter.
One of the first results also suggests a simple antonym for "fragmenting" that I had previously overlooked: "non-fragmenting".
Based on searching for the other antonyms I suggested above, I'm getting a first impression that "accumulating" and "accumulative" are probably the most common choices, although several of the others also turn up a relevant match or two. Of course, it's always possible that there's some even more common term that, not being an astrophysicist myself, I'm simply missing completely.
In any case, there are no Google results for either "accumulatory collision" or "accumulatory collisions" (although of course, as soon as Google indexes this answer, there will be).