I have a vague memory to the effect that in UF, Tolkien's oeuvre is a fictionalized chronicle of events that happened in Alfheim's historical past, he having had some sort of opportunity to visit. The only evidence for this that I can bring to hand right now is the character of Peregrine Took, appearing in Out of the Blue. If there's anything more concrete it would probably be somewhere in Featured Documents or else one of the Ask Gryphon Anything threads; I do not feel up to digging for it right now.

> Games Workshop's properties have no place in UF

I tend to agree; to import WH40K anything into the UF-verse, you would either have to turn the grimdark knob way down on the imports, at which point they'd just be another entry in the rogues' gallery ("yeah, the Klingons have had a low-grade war going on with the Orks since forever, both sides seem to like it that way"), or else you'd have to turn the grimdark knob way up on UF itself, which I doubt anyone in the writers' room has any interest in.