How can I install a 1 1/8" to 1 3/8" Synapse fork onto a 44 mm headtube?
A lower akin to an EC44/40 could be made to do this, but it doesn't exist commercially. In other words it would be an EC44 frame interface and take the sort of bearing that goes in IS49, whereas EC44/40 does the same but with IS52. This hypothetical lower can't be described in SHIS because Cannondale 1-3/8 forks all have integrated crown races.
So yes, but it would have to be a custom made cup, i.e. turned out on a lathe just for the purpose. Using EC44 on a bike intended for ZS like this will of course slacken the geometry.