As the title states: Is it possible to imbed any ring as a sub-ring of a semi-simple ring? It seems to me that this question breaks down to asking whether or not for any ring $R$, there exists a semi-simple ring, say $S$, in which $R$ and all it's subrings have compliments.
Now, I have no counter-example currently but intuition tells me no and I don't see an obvious way of constructing one.
Since you're talking about complements, I suppose you're talking about "semisimple (Artinian) rings" and not merely rings with zero Jacobson radical.
No: it is not possible. The first thing that occurs to me is that a semisimple Artinian ring is Dedekind finite, and that passes to unital subrings. Dedekind finite means that $xy=1\implies yx=1$.
So, if one chooses a ring which isn't Dedekind finite, it can't be a subring of any Dedekind finite ring (and Artinian rings have that property.)