but I don't know how to convert the set of hex digits the user inputs into memory into the format required for the FAC
I have a strong feeling that this is not about hex, but binary integers.
I know about FRMNUM ($DD67) and PRNTFAC ($ED2E), but I can't find the subroutine that Applesoft uses to take a decimal number and convert it to the six-bit exponent-mantissa format.
You're already on the right track. For example GIVAYF ($E2F2 *1) converts a 16 bit integer held in A/Y (High/Low) into a floating point number in FAC.