Thank you Lee for the reply.
In fact, that's what I've been doing so far.
One can also do it using the .net api, but I was looking for a lisp function to add a block definition directly to the table without inserting and then erasing it (maybe I should have been more clear about that in my first post).
Perhaps Irneb's reply is what I was looking for: the answer is "no", at least there's no straightforward & simple way to do it in lisp.
Thanks to both of you.
Greetings.