\definelinenote[whatever]

\starttext

    \startlines
        footnote 1 \footnote{one}
        footnote 2 \footnote{two}
        \setupnotation[footnote][start=40]
        footnote (40+1) \footnote{three}
    \stoplines

    \startlinenumbering
        dummy line \par
        dummy line \par
        \dorecurse{10}{line \the\numexpr2+#1\relax\space (\footnote{footnote #1}) (\whatever{whatever #1})\par}
    \stoplinenumbering

\stoptext
