qcc was the only package I found with an actual process.capability() function, though. In addition to its CRAN page, and GitHub repo (below), qcc also has a pkgdown site with a nice Getting started vignette.
Edit: Note, I edited the title of your question to use lowercase letters for xbar. R is case sensitive, so if you were actually typing in XBAR, it would not be able to find the xbar() function.