hmm ok...

function activate_me()
{
if (event_type == EVENT_IMPACT)
{
if(key_space)
{
//activate entity
}
}
}

action entity()
{
my.emask |= ENABLE_IMPACT;
my.event = activate_me;
}

now when the player collides with your chosen entity, it will run activate_me. If you press space while near the entity you can then run some other code.