Hi,
Thank you for posting your query on Symantec community.
Q: Can I import the additional license into SEPM to keep the license count correct? The existing and new licenses do not expire on the same dates.
--> You can do it, but make sure to renew license when it gets expired.
Refer this guide as well:
Licensing Symantec Endpoint Protection
http://www.symantec.com/docs/HOWTO55056