Overwrite Rule using Lowercase Letters?

I am trying to find a way to crack a 12-digit lower case alphanumeric password (that might take less than 10 years). I was thinking that I could build 12-digit numeric word-lists and use the overwrite rule to add letters to numeric word-lists. The majority of the password consists of numbers. A numbered word-list would also take up less hard-drive space.

Is this possible and can anyone point me in the right direction for more information on creating rules?

## rule: Overwrite characters in a wordlist with lowercase letters
## limits: password equal 12 chars in length
## letters used one time each and not repeated
## letters not placed next to each other (separated by at least one number)
## no more than 2-5 letters used at a time per 12-digit password

## example: 642394959797 ---> 6423949h9s97

#change each char to the letter "a" one by one
o0a
o1a
o2a
o3a
o4a
o5a
o6a
o7a
o8a
o9a
oAa
oBa

#change each char to the letter "b" one by one
o0b
o1b
o2b...

Rule based attack information on the Wiki:
https://hashcat.net/wiki/doku.php?id=rule_based_attack

PS I tried to bruit-force with a custom character-set. It was impractical as far as the amount of time it would take to crack.  

hashcat64.exe -a 3 -m 2500 -1 ?d?l "C:\Users\Desktop\hashcat targets\test.hccap" ?1?1?1?1?1?1?1?1?1?1?1?1