There will be many skills, called unknown skills, that your character does not have the knowledge to use skillfully. However, this does not mean that the skill cannot be used with a lower chance of success.
Associated with each skill is a default usage level. This will be one of 1/2, 1/4, 1/8, or None. If it is None, the skill cannot be used by default. Otherwise, the default skill level is calculated the same as the skill level would be (using primary and secondary attributes), and then multiplied by the default usage level.
There is nothing stopping you from recording all unknown skills on your character sheet along with your default score, apart from a lack of space. If there are unknown skills you expect your character will be using frequently, these should certainly be recorded, to save time during the game.
If the unknown skill your character wishes to use is related to a skill your character does know, the default usage level is increased by one step (e.g. from 1/4 to 1/2 or from None to 1/8) to reflect the similarity to something your character is used to doing. This is especially common with weapon skills, as there are large groups of related weapon skills.