How can I write roman numbers in the program inkscape? Do I have to use the glyph-mode somehow or how is it done? Strangely, manually putting in the Unicode for example for the 'I' , U+2160 does not work and give me an 'I' in the text on my drawings.
I would use whatever glyph finder ships with your OS and copy and paste as @Wrzlprmft suggests.
Inkscape itself has a character/glyph dialogue under
Text -> Glyphs... but on my (Xubuntu) system I couldn't find any Roman Numerals despite you giving the Unicode for one example.
"Character Map" in my "Accessories" under Xubuntu does show me Roman Numerals and I know that Windows ships with a similar Character Map program.