is it possible to save just the primary key of an object reference?

I have a scenario where tables are arranged and mapped as follows

TableA -> TableB(with primary key TableC)

to populate TableB I create an instance of TableB and set it's id  using an
instance of TableC and the persist C

What I'd like to do is persist just the pointers to TableC (which will also
later become pointers to TableB when I create a TableB instance)

How would I map that? Since currently the mapping is from TableA->TableB
there is no way to assign TableC instances to that collection on TableA so
I'm having to save both TableB instances and TableC instances at the same
time. The problem for me is I want to fill in TableB and only want to
populate the association now.

To clarify..
Let's say I have 3 tables..

table A
table AxrefB
table B

and I also have table C which a foreign key to the primary key of A and B
i.e: A(Cpk1), AxrefB(Apk,Cpk2), B(Cpk2)

What I want to do is have a relaxed foreign key constraint so I can save
AxrefB before saving B

So the process would be 
1) I'm saving an A so I create a C1 and assign it to A
2) but A also has a relationship to B so to save that relationship currently
I'd have to create C2 and assign it to B and then assign B to A's collection
and when I save A, B would also be saved.

What I want to do is create C2 and somehow just persist AxrefB.
Then later when i'm persisting B I look for C2 using other criteria that
relates B to C2 and then assign C2 to B and finally save B.

So is there a way to map this?