Custom brute-force attack

[mika09]

I have this attack:
hashcat -a 3 -m 0 "hash.txt" ?a?a?a?a?a?a?a?a?a

But I don't want to test these masks. First, I want to convert them to MD5, then take only the first 20 characters and finally add at the beggining //.

For exemple, for ?a?a?a?a?a?a?a?a?a = 123456789:
    First: MD5 (123456789) -----> 25f9e794323b453885f5181f1b624d0b
    Then: 20 first characters -----> 25f9e794323b453885f5
    Finally: Add // -----> //25f9e794323b453885f5, this I want to test to crack the password, no the mask 123456789

Create a dictionary is not not an option, to big...

So, ¿How I can proceed? I reviewed the documentation but I didn't find how to do it.

Thank you.