FIR Filter Design - input parameters conversion

I am new to DSP and filter design. I have developed a code in C++ to calculate FIR coefficients using Parks-McClellan algorithm. The inputs to calculations are:

1. Filter type (Low-Pass, High-Pass)


2. Passband Freq [Hz]


3. Stopband Freq [Hz]


4. Passband Ripple [dB]


5. Stopband Attenuation [dB].

I got a new requirement for inputs, namely to use: 1. Cutoff freq (6dB@1.6kHz) 2. Slope [24dB/octave].

Is there a way to convert these inputs into the inputs I used in my C++ code, as listed above?

Or, more general question, how to design FIR filter with custom slope?

Thank you.