I have a Samsaran Wizard SpellSlinger in Lv 8, and i want get early the NAME BULLET spell, the Wizard get it in LV 4 Slot Spells, but a want pick the same spell from the Ranger spell slot(the ranger get this spell in lv 3 slot spell), can i use the racial trait Mystic past life to get this spell from ranger list even if i already have the spell in my list(but in higjerlevel)???
No you can't.
You can add spells from another spellcasting class to the spell list of your current spellcasting class. You add a number of spells equal to 1 + your spellcasting class’s key ability score bonus (Wisdom for clerics, and so on). The spells must be the same type (arcane or divine) as the spellcasting class you’re adding them to. For example, you could add divine power to your druid class spell list, but not to your wizard class spell list because divine power is a divine spell. These spells do not have to be spells you can cast as a 1st-level character. The number of spells granted by this ability is set at 1st level. Changes to your ability score do not change the number of spells gained. This racial trait replaces shards of the past.
A spell isn't inherently arcane/divine (/psychic/alchemical), the type of spell is determined by the caster. In this case the caster is a divine caster (it's the ranger list) so you can't do it. I do expect people to disagree with me on thi point though.
From a GM perspective this screams of cheese and I possibly wouldn't allow it even if it's legal unless I think your character needs a buff.
Bloodragers/Paladins/Rangers often get spells at much lower pevel than other caster because the Character Level they need to access the spell is higher. In this case a Wizard can take this spell at level 7, while a Ranger doesn't get access till level 10. Using this method would give you access as early as level 5. I realise you're already past the point where you can cast it, but it would allow you to cast the spell more times per day, and (more importantly) it would be competing with level 3 spells for slots.
As always talk to your GM, but as far as I can tell the answer is "No".
Ranger gets 3rd spell levevl at 10th. Wizard gets 4th level casting at 7th.
I really don't understand why you can't grab Name Bullet and why you need to cast it at a lower level than needed.