synchronizing expired subscription and publication databases

The expired subscription will need to be reinitialized from the publication snapshot. Few ways to not lose the subscriber data (not yet synchronized to Publisher) is to 1. store the data in temporary tables and then after reinitializing, move the rows from the temp tables to the subscriber and then sync again.
2. Re-create the subscription with no-sync option.

Also please refer to BOL topic:
A Merge Subscription Has Expired and Changes Must Be Uploaded
under:
Configuring and Maintaining Replication --> Troubleshooting Replication --> Troubleshooting Guidance for Replication Issues