Use the \c
escape to "connect" lines of text.
From the Troff User's Manual (CSTR 54,http://troff.org/54.pdf):
4.2. Interrupted text. The copying of a input line in nofill (non-fill) mode can be interrupted by terminating the partial line with a \c. The next encountered input text line will be considered to be a continuation of the same line of input text.
.BI USB num :: manufacturer :: product\c
.BI :: serial\c
.RB [ ::\c
.IB type ]
This should work as long as the formatter is a real *roff and not a script that just recognizes a few common macros.