Can I append to a preprocessor macro?

Is there any way in standard C—or with GNU extensions—to append stuff to a macro definition? E.g., given a macro defined as
#define List foo bar
can I append bas so that it List expands as if I’d defined it
#define List foo bar bas?

I was hoping I could do something like this:

#define List    foo bar bas

#define List_   Expand(List)
#undef List
#define List    Expand(List_) quux

but I can’t figure out how to define the Expand() macro so it’ll do what I want.

Motivation: I’m playing with discriminated/tagged unions along these lines:

struct quux_foo { int x; };
struct quux_bar { char *s; };
struct quux_bas { void *p; };

enum quux_type {quux_foo, quux_bar, quux_bas};

struct quux {
    enum quux_type type;
    union {
        struct quux_foo foo;
        struct quux_bar bar;
        struct quux_bas bas;
    } t;
};

I figure this is a good place for the X-macro. If I define a macro
#define quux_table X(foo) X(bar) X(bas)
the enumeration & structure can be defined thus, and never get out of sync:

#define X(t) quux_ ## t,
enum quux_type {quux_table};
#undef X

#define X(t) struct quux_ ## t t;
struct quux {
    enum quux_type type;
    union {quux_table} t;
};
#undef X

Of course, the quux_* structures can get out of sync, so I’d like to do something like this, only legally:

struct quux_foo { int x; };
#define quux_table quux_table X(foo)

struct quux_bar { char *s; };
#define quux_table quux_table X(bar)

struct quux_bas { void *p; };
#define quux_table quux_table X(bas)

(Well, what I really want to be able to do is something like
member_struct(quux, foo) { int x; };
but I’m well aware that macros cannot be (re)defined from within macros.)

Anyhow, that’s my motivating example. Is there a way to accomplish this?

Boost.Preprocessor examples are fine, if you can show me how to make the X-macro technique work with that library.

Effectively, no.

Macros are lazily evaluated. When you #define List_ Expand(List), its replacement list is the sequence of four tokens Expand, (, List, and ). There isn't any way to expand a macro into a replacement list.

All macro replacement takes place when a macro is invoked.

I'd recommend looking at using the Boost.Preprocessor library for automatic code generation. It's a bit of work, but you can accomplish some fairly impressive things using it. It should be fully compatible with C.