I actually never understood what those umli are supposed to be and I definitely have never read something Tolkien-official about them. I always felt they did not want the dwarfs to be the only non-halfbreed in their game, as all other species, including trolls and orcs have their half-human cousins.
However there is an extinct kind of dwarfs the "small dwarfs" ( that would be the Translation back from German ). They were even smaller than ordinary dwarfs. One of them appears shortly in the Silmarillion and he is actually the last one of them. It is something about Nargothrond if I remember correctly. But he was treacherous and killed for it by an Elf. That may have been an Umli, although he was never called like that. So I do not think it is really official.
We came in peace, for all mankind - Apollo 11