Question: Inserting new entries in a table

I have a set X={a,b,c,d,e} and a table as follows:

M[2]:=TABLE([e = {2,3}, a = 5, c = 1]);

Afterwards I want to add to this table all those elements (e.g. b={3,4} and d=4) of X which are unassigned in M[2]. How I can do this?

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