In general, you can just go to View>Syntax>Open all with current extension as... and then pick whatever you want.
When you do this, Sublime Text creates a new file in your [sublime packages]/User folder with the name of the syntax and the extension ".sublime-settings." For example, when you pick "HTML (Rails)" from the "Open all with..." list, ST makes a file called "HTML (Rails).sublime-settings".
The contents of this file are pretty simple:
However, you'll note that Sublime Text only grabs the filename after the last period. If you change this file so instead of "erg" it says "html.erg", it won't work, for the same reason that it says "erg" in the first place: as far as I can tell, Sublime Text only considers the part of the filename after the last period to be the extension, and obviously "html.erg" cannot come after the last period because it contains a period.