ouch… 37 reads no answer :(

I don't know what kind of typeface you designed but a possibility is to make it lighter first and then make it bolder again using round corners. This will change te design of the glyph.

Pieter

yes I already tried that. My font family is a sans serif but I have an hairline weight that does not support the bold and light operation, because it's too thin :(

You could start your hairline weight as an unstroked path.
However, the "Expand path" command is a little cranky.
I've found that it's necessary to "Break Contour" at every point before expanding, in order to get a smooth effect, and even then some clean-up is required after "Merge contour".
If anyone knows of a better way....

nick, if ever i want to expand a single line, i always prefer to do it in illustrator.

Me too I will do it in Illustrator. And as I already draw the weights I don't feel like doing it all over again. My problem sounds to be unsolvable in Fontlab. I hope some day someone will write a little plug in (or a little script) for Fontlab to create this kind of effect. I think lots of people would like to have the opportunity to do it in fontlab and I think it is a big lack from the soft. Illustrator have it since (almost) the very begin…