Encoding Mpeg2 with captions for broadcast

The network is no longer doing our closed captions for us so I'm truing to figure out how to get our deliverable through compressor with their mpeg specs along with the captions. The captions are being made out of house by an online company.

Here are the networks delivery specs:

.mpeg file extension Mpeg2 Transport streams in 720p

Constant nitrate of between 12 and 20 mbps

Closed GOP size 15 IBBP

4:2:0 Chroma Sampling

384kb audio at 48khz

What I did was modify the mpeg transport stream preset. But I have been unsuccessful in getting the constant nitrate option to not be greyed out and I've also been unable to force compressor to spit out an .mpeg instead of a .m2t. Also, when I check the resulting file after pointing compressor at my .scc file for encode, I only see a video stream in the file. I look at it using the latest version of VLC and I can't see an audio stream or the caption stream.

Is what I'm trying to do possible with compressor 3.5 or do i need to upgrade?