Hello Pierre,

get the script get NakedEdges from here: http://kyticka.webzdarma.cz/3d/moi/#SelectNaked.
Copy the string and put it under Options/Shortcut Keys in the right field - Then choose on the left field a hotkey. Close Options. Hit the hotkey an press join. After the curves are joined and still in Highlight -press extrude.

Have a nice day
Karsten