I have not tested, but would think you can use the setvar column in sipusers table to set the WEbRTC specific parameters. You can use semicolon in the setvar column to separate the parameters, such as: avpf=yes;force_avp=yes;icesupport=yes
These webrtc parameters are built into to the pjsip tables, by the way, but if you need to stick with chan_sip then hopefully that info points you in a good direction.