I am a fairly new Sprutcam users so this may or may not be the correct answer. From what I have read and experimented with it looks to me as if you need to use two operations. One operation will use Hole Machining to do the plunge, and then a machining type operation to actually mill the part. It sounded goofy to me but that was my take on the process.
I believe there was a post (maybe on the SprutCam forum) talking about a similar issue. I think someone from that post forwarded a request to SC for an improvement in a future version of software.
I will be trying something like this shortly so if you find an answer please let us know the solution.