You cannot add another value to the Distribution Types. These values are hard coded and are used in processing.
These values are associated with the various Fund Class default accounts and are not user definable. For example, choosing Gift-Principal on the Distribution radio will cause the GL accounts in the Gift Debit Principal Asset and Gift Credit Principal Revenue accounts from the Fund Class record to populate the General Ledger tab on the unposted Gift.
Note that changing the accounts on the General Ledger tab of the Gift will not cause the Distribution Type to change. Changing the value on the Distribution tab might cause the accounts on the GL account tab to change.