Yes, Fm7b5 is a substitute for F#m7.
Fm7b5= F - Ab(G#) - Cb(B) - Eb(D#)
B = B- F#-D#(Eb)

Notice the b5 of Fm7b5 is them same note as the B of the B chord. Also note that the 7th of Fm7b5 is the same as the 3rd of the B chord. The F and/or the Ab(G#) modulate to the F# of the B chord.

JonD and I like playing chord games like this. That is what non-conventional chord can we place next! We try to have one note that is the same in both chords, however sometimes it doesn't happen.

I hope this helps.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware