You'll have to read the data sheet to understand what those ADC values represent (for example, do they represent a bipolar value or single-ended?). It's possible that a simple linear transformation (multiply the value by a number and add another number) will convert it to a range that is suitable for the DAC. You then just pick two points to find the constants of the transformation. Example: suppose the transformation is y = m*x + b where x is the ADC reading and y is the ADC value. If you want x = 0x8020 to be the DAC value of 0 and x = 0xffff to be y = 0xd54, you just write
0 = m*0x8020 + b
0xd54 = m*0xffff + b
and solve for m and b.