Very, very, very unlikely it would ever work out like that. Perhaps, maybe, if you only work for UPSF as a casual, not a regular employee. There are two kinds of hires into package car driving: (1) 'INSIDE' hires - the part time package handlers , and (2) 'OUTSIDE' hires - under which category the part time supervisors also fall. There is a (minimum) ratio of 6 INSIDE to 1 OUTSIDE hires that is maintained.
You're not going to transfer from UPSF to UPS package, you'd have to quit and try to get an 'outside' hire slot. About the best thing I would suggest is working as a dock casual for another carrier and package handler for UPS for a couple of years, as long as the shifts allow it. Or just go ahead and work full time dock/city/road for another freight co, apply for outside hire slot. By the way, no CDL required for package car driving (to my knowledge) because they're under 26,000 GVW/hydraulic brakes. And remember if you are working (driving) with a CDL, you've got to watch your hours of service eligibility.