once能不能作为状语从句的连接词？

可以。
可以引导条件状语从句，表示“一旦”。
例如：Once I killed you, you are not able to live on.

you'll become my gf,once i kiss you~

可以,意思是一旦
Once I fall in love with you , I will do anything for you

时间状从，条件状从