\setupbodyfont[dejavu]

\startluacode
    fonts.handlers.otf.addfeature {
        name = "kerntest",
        type = "kern",
        data = {
            A = { B = -500 },
        }
    }
\stopluacode

\starttext

\definecolor[tgray][s=.5,t=.5,a=1] \showfontkerns \showglyphs

\definefontfeature[kerntest] [mode=node,kerntest=yes]

\definedfont[file:dejavu-serif.ttf*kerntest @ 48pt]

\startTEXpage[offset=10pt,foregroundcolor=tgray]
    ABCDEF
\stopTEXpage

\stoptext