To the best of my knowledge, you have to go the long way round, using GetPositionFromLocation to extract the vector, and the Location function to build the new location from the modified vector.
The vector structure certainly has fields .x .y and .z of type float. I don't think the location structure format has been published, nor is it trivial to deduce the field names. I imagine it might well contain a field of type vector.
Incidentally, if anyone does know the format of the internal structures, it would be useful to know.
EDIT : What Shadooow said.