Condition Functions
 

How I can create new condition functions as in_array() is_array() is_numeric() isset() empty() defined() array() can_moderate() can_moderate_calendar() exec_switch_bg() is_browser() is_member_of() ??

Huh? Perhaps the manual will help - Using PHP Functions in Template Conditionals

Yes, I know it, thanks. But how can I create a new function as isset() that do what I want?

What do you mean create a new function? You just write one and add it (via a plugin is usually the easy way).

hook_location "whatever":
I'm just not getting your question.

If I understand you correctly, you want to add your own function to the allowed functions list?

exatly

Use the hook template_safe_functions, and add to the $safe_functions array. Note that should not be used excessively, and should only be used if absolutely required.