typedef struct 
{
   var health;
   var whataver;
}DETAILS;

PANEL* player_pan;

function build_player()
{
   player_pan = pan_create(...);
   player_pan.skill_x = sys_malloc(sizeof(DETAILS));
   ((DETAILS*)player_pan.skill_x).health = 100;
   ...
}

function player_hurt()
{
   ((DETAILS*)player_pan.skill_x).health -= 5;
   ...
}