they add a 0 to the Radix-4 number and add a 'Z'
The radix number is non-negative, but a complement number starting with 3 in base-4 is negative. By adding the leading zero we make sure that it is still non-negative. In general radix numbers can be extended with leading zeroes, without changing their value. Complement numbers that are non-negative can be extended with leading zeroes. And complement numbers that are negative can be extended with B-1.
Is this solution sufficient?
If it says "give the computation steps" and "don't use base-10 as intermediate", you would need to provide more. But if not, then this solution is ok. In any case it is useful to write some of the computation steps. That way you can gain partial points even if the final answer is wrong.