I just simply tie the end of the messenger line to the existing halyard by opening up a hole in the end with my fid, then tying a long boline. never had a prob with this method.
Splicing is the only way to go. a knot can reduce the strength of the halyard by a large percentage. make sure that whoever does your splicing uses a core-to-core splice so that you have 100% of the line strength.