How to go back to last word end

Dear GNU roff users,
I'm having an issue writing a simple macro.

This is the code for my bold macro

.de B
.  ft B
\\$*
.  ft
..

which works quite well, except when I want to add a punctuation mark
right after the last bold word.

.B my bold words
,
not so bold words

This would be my desired output

*my bold words*, not so bold words

But I can't find a way to achieve it consistently. The best I could find
was

.B my bold words
\h'-\w'\~\~'u'
,
not so bold words

However this doesn't work when groff adds adjustment space, when it's in
b or n adjust mode.

Is there a way to go back to the last word end, before the space?

And if there is, is it portable to other roff implementations?

-- 
Matteo Bini