Can I programmatically copy sources and citations from one Ancestry.com tree to another?

On Ancestry.com, I can choose an ancestor and save the person to another tree. However, the citations and sources are not copied over. I'm writing a program to do that but have a question on SOUR xref uniqueness.

From what I understand, xrefs only need to be unique within the tree. If that's the case, can I generate my own xref for a source then manually insert those lines into the target tree's .ged file? Does Ancestry use another id in addition to the xref I need to be aware of?

// This is the repository.
0 @R1234567890@ REPO // Repository xref
    1 NAME // Name

// This is the source. I would get these lines from 
// the source tree .ged, change the repository to one
// that exists in the target tree, then insert 
// them into the target tree .ged.
0 @S1234567890@ SOUR // Source xref
    1 TITL // Title
    1 REPO @R1234567890@ // Repository xref
    
// This is the source citation. I would transfer
// these to the target tree as well.
0 @I1234567890@ INDI
    1 NAME Full /Name/
        2 SOUR @S1234567890@ // Source xref
            3 PAGE // Detail
            3 DATA
                4 TEXT // Transcript