No such option exists in GDAL's DXF driver. Extents are calculated
according to the overall bounding envelope of the geometries.
DXF files are not difficult to manipulate as plain text, so if you
wish to modify the extent values (which are conveniently stored very
close to the beginning of the file in the $EXTMIN and $EXTMAX
variables) it should not be terribly difficult to write a simple
Python (etc) script to do this.