sfsegyread currently limits the number of trace header keys to 71. However, you can remap a non-standard header. For example,

sfsegyread < file.segy tfile=tfile.rsf swdep=188 > file.rsf

will map the header value at 189-193 bytes to the “swdep” standard header.
Note that numeration starts at 0 at and that the number of bytes in the
headers (2 or 4) should match. You can test the output by running

sfheaderattr < tfile.rsf