Some systems do not support User-Select or Contact Select custom fields. 

The following query will transform User-Select fields into Matter Participants.

This script can easily be adapted for Contact-Select fields.

-- Transform Custom Fields on Matters that reference Users
-- into Matter Participating Users.
INSERT INTO __M_Matters_Participating_Entities (
    Id, 
    Final_Matter_Id, 
    Final_Participant_Id,
    Final_Participant_Type,
    Final_Participant_Role
)
SELECT
    CONCAT('TRANSFORM --- ', V2.Id),
    V2.Final_Parent_Id,
    V2.Final_Value,
    V1.Final_Kind,
    V1.Final_Subject
FROM
  __M_CustomField_Definitions V1,
  __M_CustomField_Values V2
WHERE 1=1
  --Change this to '__M_Contacts' if you want to do this for contact custom fields
  AND V1.Final_Kind = '__M_Users'
  AND V1.Final_Parent_Type = '__M_Matters'
  AND V1.Id = V2.Final_CustomFieldDefinition_Id