Hello, sorry for the late answer.
you cannot do this, since by definition the flow is exiting the domain, thus the value at the boundary dependent strongly on what is going out from within your domain.
what you could do, is to set the value at your boundary. TELEMAC will change it though, but it will also use your imposed as well.
Note that this is not conservative -- thus why we cannot allow it in the standard version.
Hope this helps,
Sébastien.