Is there any term for the habit of playing around with words, or even names, and nickname them? Like 'No' becoming 'nopes', 'whatever' becoming 'tevs', milky-wilky etc. This happens a lot with cricket teams, who keep nicknaming people - Wilkinson becomes Willy or Wilks, Hogg becomes Hoggy (though it's longer).
'Nicknaming' specifically for names.
Other words you can use: 'abbreviation' or 'to abbreviate' and 'dimimutive'
di·min·u·tive (adjective) /diˈminyətiv/
Extremely or unusually small - a diminutive figure dressed in black
(of a word, name, or suffix) Implying smallness, either actual or imputed in token of affection, scorn, etc., (e.g., teeny, -let, -kins)