Would I be able to have a familiar take possession of a doll or stuffed animal i have? Is this even possible? If so would I need to do this with an already existing familiar or would I need a new one?
Yes, familiars are traditionally anchored to physical objects. They don’t so much “possess” the object, as they are bound to it. It anchors them to the physical plane.
The classic witch’s black cat was actually a spirit familiar bound to the physical form of the cat.
You can do it with an existing familiar, or a new one.
I agree with DarkestKnight. You can create a familair or ritual object. The object would not be possessed but used as a portal. You should consecrate the item with your ritual. The choice of deity is up to you. Follow your heart with your own creative rite. You might have some amazing results.
You could also create a tailsman and consecrate it with ritual. That way you could carry with you, either for luck or divination.
Any books on how to anchor a familiar