From Forum Nokia Wiki

Unfortunately there is currently no Freeware signing process. The way to go is to self-sign your freeware. If your freeware requires non-grantable capabilities your only alternative is to use test range UIDs and ask the users to sign your software themselves using Open Signed Online.

Note that your application does not need to be signed by Symbian if you don't use any extended capabilities. If you are using basic capabilities only, i.e. LocalServices, Location, NetworkServices, ReadUserData, WriteUserData and UserEnvironment, you use self-signing.