I would like to know if currently VPHOT can upload comparison star sequences with Sloan magnitudes.
At present VPhot does not have access to sloan filter reference magnitudes.
You can, though, create a sequence for a target that includes sloan filters:
On the image page (SelectStar) you can click on a star and the pop up window offers "edit sequence". There you will see the BVRI magnitudes from VSD and there are also a place to input the sloan reference magnitudes.
We are actively working on making sloan reference data available via VSD.
Beyond that you can transform sloan filters to cousins if you have developed the transform coefficients.