LTM lenses have a flange to film distance of about 28.8mm. The Leica M mount is about 1mm less so that you can use an adapter to put LTM lenses on M bodies. The F-1 flange to film distance is about 42mm, so no adapter would allow the LTM lens to focus at infinity. I haven't seen one, but you might find an adapter that will put the LTM lens on the F-1 with only close focus capacity. I have seen adapters to put SLR lenses on Leica rangefinder bodies; one even has an RF activator ring that measures distance with the rangefinder that you then transfer to the SLR lens focusing scale.

This is a typical problem when putting the mirror in an SLR body. The lens flange distance and lens design have to allow for mirror clearance. That's one of the reasons that people typically claim better wide angle lens performance for rangefinders. The rangefinder wide angle lens designs don't have to be retrofocus, but can be much more straightforward designs.

BTW, while I'm typing, I don't see any reason to avoid the FD 100mm Macro for portraits. You can always degrade the performance to your desired degree with appropriate filters or other junk in front of the lens. I bet your new arrival will have soft, smooth, small-pored skin anyway. If grandma shows up for the shot with some degree of vanity, you can always throw on a filter. Then you get a very sharp macro as well, and not just a dedicated portrait lens.