You can import specific track in VSQ which is created in VOCALOID2 onto VOCALOID3 Editor. Choose [File] → [Import] → [Track] and click [VOCALOID3 Secuence (*.vsqx)] from the pull down menu to change it to [VOCALOID2 Sequence (*.vsq)].

After that choose VSQ file you wish to read, and then select the track you wish to import data to, as it will be imported as new track.

Please note that VOCALOID3 Editor has the maximum number of 16 tracks operation, so if there are already 16, you will not be able to import anything.