SOGo's Sieve support does not coexist with personal sieve scripts either created by external tools or hand written by the user or administrator. One minimal solution to coexist would be to support the "include" extension.

Example UI change:

Under the "Perform these actions" types of actions, add "Include personal filter" which would take an argument that the user could type in.

Example setting:

SOGoSieveFilters: [{"name": "Process Mailing Lists", "match": "allmessages", "actions": [{"argument": "mailing-lists.sieve", "method": "include"}], "active": 1}]

Resulting sogo sieve script:

require ["include"];
include :personal :optional "mailing-lists.sieve";