This can be done with sed
sed 's:</Directory>$:&\n\n<Directory "/">\nSSLRenegBufferSize 26215000\n</Directory>\n:g' inputfile
As it may not be obvious, here is some explanation.
The substitute option to sed is delimited with colons to avoid having to escape the forward slashes.
The </Directory>$ specifies the line to be matched has </Directory> at the end of the line.
The &\n\n<Directory "/">\nSSLRenegBufferSize 26215000\n</Directory>\n specifies that the replacement string is that which was matched (&) followed by your required replacement interspersed with new lines (\n) as necessary.
The trailing g specifies a global replacement in the file.
If this is what you want, redirect the output from the command to a file.