Players created now will end up needing 30 boosts to reach level 79. There will not really be any extra boosts you can take since 79 is the maximum level.
http://goallineblitz.com/game/forum_thread.pl?thread_id=4686077&page=1#42967169 This link explains why this is, but if you do not understand it's pretty simple. In order to reach level 79, you can only do it by boosting.
Your players level (usually around level 67) will reach a point where you stop gaining XP and only gain levels by boosting. You will be at 18 boosts at that point, and will need the last 12 boosts (total of 30) to reach level 79 (12 boosts from level 67).
So all that said, no you cannot buy extra boosts for your free player, but you will not need to. The free player gets all their boosts for free, and will be able to reach level 79 the same as any other player.