Creating them is not hard.  Getting the actual metrics sometimes is.

The AT&T documentation "Troff User's Manual" describes the DWB width
table format.  Your DWB may already have a manual page for it.

Psroff 3.0 contains documentation on DWB and CAT troff format width
tables.

DWB 3.3, Psroff 3.0 and tpscript each have a utility for automatically
creating width tables by sucking the data out of a PostScript device.

Psroff 3.0 and jetroff have utilities for creating width tables from
HP laserjet fonts.

Psroff 3.0 can convert between any of the 5 width table formats.

Groff contains a package for generating groff width tables from HP's
scaleable font metrics format.

Consult the documentation to find out where to put the files.

Some parts © 2009 Advameg, Inc.