Thanks for your answer.

You can name them 11.00, or 10.99 if you prefer to have them "linked" to the 10th season (and also use 10.98 if there are two of them).
And indicate in the comments of the movie/series page what is that special number.

Greetings to all,
I’m asking you about naming episodes "Off-Series" for a series.
To explain my case, in the series "Candice Renoir".
http://www.imcdb.org/movie_2819924-Candice-Renoir.html

There are two "Hors-Serie" episodes of 1h30 each between season 10 and season 11.
https://fr.wikipedia.org/wiki/Liste_des_%C3%A9pisodes_de_Candice_Renoir

And my question is: How numbered are the vehicles after season 10 and before season 11?
I tried "10.HS1" but they go before season 10.
Thanks for your help.