[Note: I've heavily edited this post.]
Hi
balise75!
The font that I use is Garamond.
Ok, so, first off, Unicode defines three different specialized space characters of nominally thinner widths than the standard space character. They are:
U+2008 Punctuation Space: Space equal to narrow punctuation of a font > <
U+2009 Thin Space: A fifth of an em (or sometimes a sixth) > <
U+200A Hair Space: Thinner than a thin space; in traditional typography, the thinnest space available > <
In the listings above, the space character between each of the >< corresponds to the character described. You should be able to highlight that character and then copy-n-paste it.
However... I just tried using each of the thinner spacing characters described above in Scribus, and they all are rendered as the standard space character. Even going the route of Insert > Glyph doesn't accomplish anything.
What I'm going to suggest is that, at least for purposes of Scribus, you simply define the appropriate width by way of taking a standard space character and then changing the width manually, and then copy-n-pasting that into each instance of usage. It's not really ideal, but I can't find another way to get what you might be looking for.