Using sed(1) to modify EOL chars is, AFAIR, only possible iff you've accumulated a number of lines (by appending them in, using the H command, to) the hold space e.g. the following will cause all lines in file to be output on the same line:

# sed -n 'H;$x;$s,\n,,gp' file
       ^  ^  ^  ^
       |  |  |  |
       |  |  |  |
       |  |  |  For the last line only, globally strip all line ending
+s in the pattern space and print the result
       |  |  |
       |  |  For the last line only, swap the hold and pattern spaces
       |  |
       |  Append each line to the hold space
       |
       Don't print any lines - by default all lines are printed follow
+ing processing
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