Well actually it's a "feature"... When importing contacts from another device, we don't know how to link IM details (SIP etc.) to existing accounts. Who tells me that the SIP identifier I received is valid in a SIP account on device?
In the editor, you will never see any existing online accounts, those are visible in the "unmerge" view (not very intuitive :-/).
Not possible to send messages to a phone number: because we don't assume that every phone number is a valid SIP identifier. How would that work by the way, convert it to SMS?