Be so kind as to feed my dog.
Can this be an interrogative sentence?
im bit confused.
It is interrogative only if preceded by "Would you.." The latter form would also be much more polite. "Be so kind as to.. " can be considered a rather way of giving an order order, depending upon the tone of voice and body language.
English sentences essentially fall into four categories:
declarative -- normal sentence, giving information, stating facts, etc.
interrogative -- asking a question
imperative -- giving a command
exclamatory -- sudden, strong statement, but may be missing subject or verb
Which one do you think you have here?
??