Hi Katherine - If you mean the .trk format, that's not for storing labels, it's for storing tractography streamlines (i.e., a bunch of curves that are stored as sequences of x,y,z point coordinates). You can convert the FreeSurfer labels to nifti volumes, and see if the software that you're trying to use for graph analysis can read in the labels in that format.
There are two different inputs that you need for your graph analysis, the tracts in .trk format and the labels that will be the end points of the tracts - you have to find out what the format of the latter needs to be.
a.y