PreSonus just fixed this with Studio One 5. You still have "add instrument track" that you can use first to setup the track and name, but now there's a macro command called "add instrument to selected track" with the dialog box that allows you to choose from all your 1st & 3rd party VST Instruments and any custom presets that you have created for them available as well.
I have checked version 4.6 for it and though it will show your custom macro with the new command, it is not in the macro command list and cannot be accessed, which is a bummer for those who have version 4, but at least we've been heard and they put it in v5. I upgraded [Studio One 4] => [Studio One 5] hoping that they fixed this issue and they did!
I'm so happy right now because my song template was dependent on this and before I had to create a long drawn out workaround using the "Browser" & "Navigation" Macro Commands which involved opening the browser to the instruments tab and navigating down 50 x's just to get to the VSTi I wanted to load and then have it navigate back to the top so to reset the initial browser focus position, etc... The new macro fixes that in less than three commands and I don't have to worry about fixing my work around version every time I add new instruments, so yeah, get v5 and enjoy brother.
[ And if you guys & gals @PreSonus see this, THANK YOU SO MUCH ! ! ! ]