Hi Laura,

I do not see an easy solution. Although JanusGraph supports custom vertex id's, I do not belief this is compatible with the gremlin io readers (at least, not out of the box, I tried...).

An alternative collaboration model would be to setup Gremlin Server. Then you have the gremlin language variants available (e.g. python) to write new and modified data directly to a shared graph (without using graphML files for transport). Apperently, you have an external naming convention to recognize shared vertices, so you could add the external names as properties and define a janusgraph index for that.

Best wishes,     Marc