This isn't a complete answer, but searching Google Scholar for "Fibrobacter succinogenes human" I found this paper:
The cellulose-degrading microbial community of the human gut
varies according to the presence or absence of methanogens
The abstract of which begins this way:
Cellulose-degrading microorganisms involved in the breakdown of plant cell wall
material in the human gut remain rather unexplored despite their role in intestinal
fermentation. Microcrystalline cellulose-degrading bacteria were previously identified
in faeces of methane-excreting individuals, whereas these microorganisms were
undetectable in faecal samples from non-methane excretors. This suggested that the
structure and activity of the cellulose-degrading community differ in methane- and
Note also in Table 1, Fibrobacter succinogenes is one of the strains that seem to have been detected, so that answers the part of your question that seems to ask if humans can incorporate those bacteria into their gut fauna: yes, some already do.
This suggests to me that:
1) some humans do have a limited ability to digest cellulose through their gut bacteria, though this ability is probably limited enough that "humans can't digest cellulose" can still be considered true in the large scheme of things,
2) the challenge in digesting cellulose isn't so much getting the gut bacteria in there, but providing them with an environment where they will thrive and digest a lot of cellulose. This is probably where the specialized gut structures of ruminants come in; it becomes not so much a matter of digesting cellulose, but of digesting it well enough that you can live off of it. I'm guessing this is also related to the differences the paper found between people who excrete methane and those who don't but I wasn't able to figure out what that was supposed to be related to (is it a cause or an effect of the presence of certain gut bacteria? is it related to diet, genes, something else? I couldn't find that information in the paper though I didn't read every word)
3) according to the paper there isn't much research done into this.
4) the paper gives no info as to how significant those cellulose-digesting bacteria are to the host's energy balance, which I imagine is what you're really interested in. Is it significant, is it too small to be noticeable, does the host even benefit at all calorie-wise? They don't address that question in the paper.