In Vixen I copied my old Profile and reserved the first 512 channels for my Renards then added another 2700 channels for PixelNet using the Add channel in the Edit a Profile for a total channel count of 3212.
I then created my PixelNet channels in VSB using the 3212 as the total channels and the SSC start channel as 513.
In the Old Profile under <ChannelObjects> I cut from output 512 (because the profile output channel starts at 0) to the end of the channel list (</ChannelObjects>) and pasted the same range from the Vixen file created by VSB.
This preserved all my old profile settings for the first 512 channels and added the new PixelNet string settings to the end of it.
You should practice with adding one or two strings until you get the hang of it, then go for it.
It is a real PIA to get everything lined up but I was able to use the new profile with all of my old sequences and add in the PixelNet channels.
In VSB you can also select different sort criteria and later paste these into your profile as well.
Please make backup copies of your profile because it's very easy to screw this up and you always want to be able to start over. I learned the hard way, but got lucky because I had versions all over the place.
Good luck
Ron