When else fails, read the documentation:
LaTeX-math-list allows you to add your own mappings.
A list containing user-defined keys and commands to be used in LaTeX Math mode. Each entry should be a list of two to four elements.
First, the key to be used after
LaTeX-math-abbrev-prefix for macro insertion. If it is nil, the symbol has no associated keystroke (it is available in the menu, though).
Second, a string representing the name of the macro (without a leading backslash.)
Third, a string representing the name of a submenu the command should be added to. Use a list of strings in case of nested menus.
Fourth, the position of a Unicode character to be displayed in the menu alongside the macro name. This is an integer value.
You can customize this option with M-x
LaTeX-math-list RET and see the following screenshot:
When you're done hit the "Apply and Save" button.
Alternatively, put this code in your init file:
'((?8 "infty" "Misc Symbol" 8734)
(?I "int" "Var Symbol" 8747)))
AUCTeX is highly customizable, you should never manually edit the source code.