As you already mentioned it's not a good idea to extend the UI5 font for future compatibility reasons. If you already have your own font you can easily register it with UI5 and reference it using a similar url-schema. Instead of sap-icon://funny-icon
you could say sap-icon://dparnas-icon/funny-icon
.
Here is a sample implementation:
jQuery.sap.require("sap.ui.core.IconPool");
jQuery.sap.require("sap.ui.thirdparty.URI");
(function() {
var aIconNames = [ "funny-icon", "another-icon" ], // the icon names
sCollectionName = "dparnas-icon", // the collection name used in the url-schema
sFontFamily = "DarnasIcons", // the icon font family like defined in your css
aIconContents = [ "E003", "E004" ]; // the unicode characters to find aIconNames under, same order
// add the icons the your icon pool
for (var i = 0; i < aIconNames.length && i < aIconContents.length; i++) {
sap.ui.core.IconPool.addIcon(
aIconNames[i],
sCollectionName,
sFontFamily,
aIconContents[i]
);
}
}());
Furthermore you will have to define the font-family in your CSS. That's it! It's easy but hard to find ;)