Fitting a predefined classification tree

Hi,

I've searched R-help and haven't found an answer. I have a set of data from which I can create a classification tree using
rpart. However, what I'd like to do is predefine the blank structure of the binary tree (i.e., which nodes to include) and then use a package like rpart to fit for the optimal splitting criteria at each of the predefined nodes.

Does such a package exist?

Thanks,
Lee