\font\eight=eight[hf,dek]

{\catcode`\.=\active
  \gdef\beginddtone{\begingroup \offinterlineskip
    \eight \catcode`\.=\active \ddconvert}
  \gdef\ddconvert{\futurelet\next\ddcontinue}
  \gdef\ddcontinue{\ifx\next.\else\let\next=\dotwo\fi\next}
  \gdef\dotwo#1#2{\char"#1#2\ddconvert}
  \gdef.. {\par\futurelet\next\ddrestart}
  \gdef\ddrestart{\ifx\next\endddtone\else\let\next=\ddconvert\fi\next}
  \gdef\endddtone\endddtone{\endgroup}}