No, FamilyTalk rate plans are limited to a max of 5 lines. You might consider splitting your current FamilyTalk plan into two FamilyTalk plans. You could for example chose the FamilyTalk $59.99 plan that has a limit of 3 lines. This plan is not available in all markets but it might meet your needs. Changes to your rate plan don't impact your individual line contracts so you do have some flexibility.
Note: as the previous poster indicated, you can have multiple FamilyTalk plans on a single account.
Dave