Hi Jack, I have not done this with a Lhires III but I did try it with a LIAS. I used a 1:1 transfer lens(two doublets) to project the exit beam from a 50 micron fibre on to the entrance of the LISA with the slit removed. It worked fine but you get a 50 micron slit.
I had an F5.4 telescope which matched the F5 LISA even if the fibre reduced the focal ratio to about F4.5
This will be the issue with the Lhires III. It is designed for F10 so if you use a 50 micron fibre you will have a large “slit”
If you feed it at F10 you will get an F5-6 exit beam and so light loss. You could match the F ratios using transfer lenses but this will increase the size of the “slit” or vice versa. You could retain the slit but then you will get more light loss.