key binding question

[Drew Adams]

I have a user variable `foo' whose value has the same form as a key-sequence
argument to `define-key'. That's all I know.

So, for example, `foo' might have value [S-iso-lefftab] or "\C-e" or [?\C-x
?] or "^E" (i.e. Control-E) or whatever might be acceptable to `define-key'.

How can I, in Lisp, bind command `bar' to `foo's key, but with the Control
and Meta modifiers added. So, for example, if `foo' = [S-iso-lefftab], then
the code would bind `bar' to [C-M-S-iso-lefttab].

If this is too difficult, suppose that `foo's value were guaranteed to
always be a vector - same question.

Can anyone help with this?