First of all, I'm not asking this question to solve an issue. My GM and I both agree on the fact this should not be possible to avoid any game breaking mechanics.
However, we couldn't find any rules on this matter on the wiki we're using (we're not native English speakers), I asked Google only to find out people saying "I think you can't" without any source, or leave the subject and speak about learning spells and not using already learnt ones.
My mage is currently level 3 with 24 intelligence, as far as I know he cannot learn 3rd level spells. Nonetheless, he has access to two bonus 3rd level slots. What's stopping him to learn 2nd level and fuse with any +1 metamagic feat (such as Intensified Spell)? Same with 1st level spells and +2 megamagic feat.
Please, use official rules based evidence. Thank you and have a good day.