。
She added milk to her coffee and stirred it.
Literal
She [topic-は] coffee [into-に] milk [object-を] put [and-て] stirred.
The に here marks the destination of the 入れる ('put in, insert') action — 'into the coffee.' Many actions of placement work this way: 鞄に入れる (put into a bag), ポストに入れる (put into a mailbox). かき混ぜる ('stir, mix together') is a compound verb: 掻く ('scratch, rake') + 混ぜる ('mix') — the 掻く element evokes the swirling motion of the spoon. The whole sentence shows clean te-form chaining where the first action sets up the second.